fix(tools): send the body for oneOf/anyOf/allOf OpenAPI request schemas

[chelsealong]

Fixes #6991

Problem

For an OpenAPI operation whose request body schema is polymorphic (oneOf,
anyOf, allOf) or has no type, RestApiTool builds a correct function
declaration with a body parameter, the model supplies that argument — and
the HTTP request is sent with no body at all. No exception, no warning,
no log line; Content-Type: application/json is still set, so the server
receives a well-formed request with Content-Length: 0.

Root cause

OperationParser._process_request_body names a polymorphic/untyped body
parameter 'body' (instead of '') specifically to avoid emitting an
empty-named property in the function declaration (this was the fix for
#2213). RestApiTool._prepare_request_params, however, only recognized the
legacy '' sentinel when deciding which parameter represents the full body:

if param.param_location == "body" and not param.original_name:

Since a polymorphic/untyped schema's parameter is named 'body', not '',
this condition never matches, body_data stays None, and json is never
set on the outgoing request — silently.

Fix

_prepare_request_params now treats both '' and 'body' as the full-body
sentinel, matching what OperationParser actually emits:

if param.param_location == "body" and param.original_name in ("", "body"):

Test plan

Added test_prepare_request_params_oneof_body, which builds a oneOf
request body schema (mirroring the issue's {card}/{iban} example) and
asserts the constructed request actually carries the JSON body.

Confirmed the new test fails without the fix (git checkout HEAD~1 -- src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py):

FAILED tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py::TestRestApiTool::test_prepare_request_params_oneof_body - KeyError: 'json'
1 failed, 63 deselected in 1.84s

And passes with the fix restored:

$ python3 -m pytest tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py -q
64 passed, 17 warnings in 1.75s

Full openapi_tool suite and the broader tools suite also pass:

$ python3 -m pytest tests/unittests/tools/openapi_tool/ -q
280 passed, 24 warnings in 3.64s

$ python3 -m pytest tests/unittests/tools/ -q
2126 passed, 1 skipped, 762 warnings in 40.31s

Formatting/lint checked with the repo's configured tools:

$ python3 -m pyink --config pyproject.toml --check src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
All done! 2 files would be left unchanged.

$ python3 -m isort --settings-path pyproject.toml --check src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py tests/unittests/tools/openapi_tool/openapi_spec_parser/test_rest_api_tool.py
(no changes)

$ python3 -m ruff check --config pyproject.toml src/google/adk/tools/openapi_tool/openapi_spec_parser/rest_api_tool.py
All checks passed!

Disclosure

This PR was prepared with the assistance of an AI coding agent (Claude),
under human review before submission.