SDK-1056: Add opt-in exception capture to the Unity Audience SDK

[nattb8]

What

• New ErrorTrackingConfig.CaptureExceptions, off by default
• When on, the SDK listens for uncaught C# exceptions via Application.logMessageReceivedThreaded, filtered to LogType.Exception only
• Each one is sent through the existing Track pipeline as a new exception_captured event, with exception type, message, and stack trace as properties
• Rate limited per exception type via a token bucket: burst up to 20, refills 1 every 10 seconds. A repeating bug settles into a steady trickle instead of going silent, and can't crowd out a different, unrelated exception type. Matches how PostHog's own client-side exception rate limiter works
• Pure C#, works the same on Windows, Mac, Linux, iOS, and Android
• Sample app: adds a Capture Exceptions toggle and a Throw Test Exception button to the Setup tab, so this is covered by the same PlayMode E2E suite every other feature is

Why

Right now an uncaught exception only shows up in a studio's own local Unity console, nothing reaches Immutable. This is the cheap first step before real native crash reporting (segfaults, ANRs), which is separate, bigger work.

Test plan

• Manually verified end to end in the sample app: forced the config on, triggered a real exception via Debug.LogException, and confirmed the actual outgoing payload contained "eventName":"exception_captured" with the right properties
• Unit tests for the rate limiter itself (burst capacity, refill over time, independent buckets per exception type, reset) using an injectable fake clock, no real waiting
• Integration tests on ImmutableAudience.CaptureException: disabled by default, consent gating, burst-then-stop, different types not sharing a bucket, refill over time, and the cap resetting on a fresh Init after Shutdown
• New PlayMode E2E tests: capture enabled queues the event and flush reports no errors; capture disabled leaves the queue untouched
• Sample app E2E CI check can be flaky, unrelated to this change if it fails, commitlint/unit tests/lint are the ones that should be clean

Ticket: SDK-1056

🤖 Generated with Claude Code

[github-actions]

Audience SDK — Build Size

Platform	SDK Size	Change
Android	0.40 / 20.00 MB	+0.04 MB	✅
Windows	0.17 / 20.00 MB	+0.01 MB	✅
iOS	8.62 / 20.00 MB	+0.10 MB	✅
macOS	1.04 / 20.00 MB	+0.13 MB	✅

SDK Size = build minus empty app. Change = vs baseline. Fails if any platform exceeds its absolute size limit.