fix: missing exports, context skip_methods, and compression "none"

[AlexeyShalaev]

Five defects that came out of writing docs/agents.md against the source. All five
reproduce; each fix is additive, nothing is renamed, removed or re-defaulted.

1. AsyncContextInterceptor could not be given skip_methods

AsyncServerInterceptor has taken skip_methods since it was written, and
docs/guide/interceptors.md documents it as "a constructor kwarg" without
qualification, but AsyncContextInterceptor.__init__ neither accepted one nor
forwarded one — it called super().__init__() bare.

>>> AsyncContextInterceptor([], skip_methods=SKIPPED_HEALTH_METHODS)
TypeError: AsyncContextInterceptor.__init__() got an unexpected keyword argument 'skip_methods'

So every health probe went through header extraction, and with a required=True
HeaderConfig a kubelet probe — which carries none of the service's headers — was
aborted with INVALID_ARGUMENT.

Fixed: the kwarg is keyword-only and defaults to (), so behaviour is unchanged for
everyone. I deliberately did not default it to SKIPPED_HEALTH_METHODS like the
four observability interceptors do: this one binds values the handler itself may read,
so skipping is an opt-in, not a new default. Two tests in
tests/unit/aio/interceptors/test_context.py (both TypeError before the change).

One correction to the finding as written: this is not "the one interceptor with no way
to opt a method out". AsyncExceptionHandlerInterceptor has no skip_methods either.
I left that one alone — an interceptor whose whole job is mapping exceptions to statuses
has no business skipping RPCs — and made the agents-page table say so instead of leaving
the gap unexplained.

2. BaseGrpcServerSettings rejected compression_algorithm="none"

COMPRESSION_ALGORITHMS maps "none" to gRPC's GRPC_COMPRESS_NONE, and
build_grpc_options has always translated it (tests/unit/test_options.py asserts the
"none" -> 0 mapping), so GrpcServerConfig(compression_algorithm="none") builds a
server fine. The pydantic model typed the same field Literal["deflate", "gzip"] | None:

>>> BaseGrpcServerSettings(compression_algorithm="none")
ValidationError: Input should be 'deflate' or 'gzip' [type=literal_error, input_value='none']

The two settings shapes are documented as interchangeable everywhere the kit accepts
settings, so this was a real hole. Fixed by widening GrpcCompressionAlgorithm to
Literal["none", "deflate", "gzip"] rather than dropping "none" from the option
builder: "none" is an explicit request for gRPC's no-compression algorithm, which is
not the same statement as leaving the field None. Parametrized test in
tests/unit/test_settings.py (fails on [none] before the change).

3. GrpcServiceName was not re-exported at the package root

grpc_server_kit.protocols exports it, the kit's own dishka providers register the
service name under it, and the root re-exported the five protocols beside it and not
this one — so anyone writing their own provider had to import from the submodule:

>>> from grpc_server_kit import GrpcServiceName
ImportError: cannot import name 'GrpcServiceName' from 'grpc_server_kit'

Fixed: exported from grpc_server_kit and from grpc_server_kit.aio (the agents page
describes aio as "everything above plus …", which only stays true if both get it).

4. grpc_server_kit.aio exported reset_signal_handlers but not the async form

aio.reset_signal_handlers_async and aio.SignalManager were both missing, though
ServerLifecycleManager uses reset_async internally, the docstrings recommend the
async form, and ServerLifecycleManager(signal_manager=…) /
run_async_grpc_server(signal_manager=…) both take a SignalManager.

aio.reset_signal_handlers:       exported=True  attr=True
aio.reset_signal_handlers_async: exported=False attr=False
aio.SignalManager:               exported=False attr=False

Fixed: both re-exported. implicit_reexport is off for mypy, so a name missing from
__all__ was not importable for type checkers either — the tests in
tests/unit/test_init.py assert the attribute and __all__ membership.

5. The quickstart was wrong about health and reflection

When health is enabled, the health service name is advertised via reflection
automatically.

Half the story. GrpcApp.build() appends grpc.health.v1.Health only when
enable_reflection([...]) already gave it a list; reflection turned on solely through
settings.enable_reflection = True fails the build:

>>> app = GrpcApp(GrpcServerConfig(enable_reflection=True)); app.enable_health(); app.build()
ValueError: Reflection is enabled but no service names are configured; call
app.enable_reflection([...]) with your fully-qualified service names

Here the doc is what drifted, not the code: reflection with no service names is a
misconfiguration worth failing the build on, and docs/agents.md already states the
rule correctly. Corrected the quickstart line. Documentation only.

Agents page

docs/agents.md changed with the API, per CONTRIBUTING.md:

• the interceptor table no longer says AsyncContextInterceptor is "the only one with
  no skip_methods argument" — it has one, and row 5 now records that the exception
  handler is the one without;
• the "note the asymmetry" sentence about COMPRESSION_ALGORITHMS vs
  BaseGrpcServerSettings is gone — there is no asymmetry left;
• GrpcServiceName moved from "not re-exported at the package root" into the root API
  table, and the aio paragraph and lifecycle table list the signal names it now
  re-exports.

docs/guide/interceptors.md gains a short paragraph on the context interceptor's empty
skip_methods default and why you would pass SKIPPED_HEALTH_METHODS to it.

Verification

$ make check
uv run ruff check .            All checks passed!
uv run ruff format --check .   110 files already formatted
uv run mypy grpc_server_kit    Success: no issues found in 52 source files

$ make test-unit
411 passed, 23 deselected in 1.69s

$ make test-integration
23 passed, 411 deselected in 0.76s

$ make test
434 passed in 2.49s — Required test coverage of 90% reached. Total coverage: 97.60%

uv sync --frozen --all-extras --group dev; uv.lock unchanged.

No changelog entry: CONTRIBUTING.md leaves CHANGELOG.md to Release Please.

Nothing here is breaking

Three added names, one added keyword-only parameter with a behaviour-preserving default,
one widened Literal, and two doc corrections.