fix(scm): treat unknown GitHub PR actions as unsupported events, not errors

[billyvg]

Summary

Fixes SENTRY-5R97 — ValidationError: Invalid enum value 'stacked' - at $.action in the GitHub webhook handler.

Root cause: GitHub introduced a new pull_request action type "stacked" that isn't in the PullRequestAction Literal[...] type in the currently pinned sentry-scm==0.32.0. When msgspec.json.Decoder(GitHubPullRequestEvent) decodes a webhook with action: "stacked", it raises msgspec.ValidationError. This exception is a generic Exception, so it bypasses the SCMProviderEventNotSupported catch in produce_event_to_scm_stream and is instead passed to report_error — creating a Sentry error for every affected webhook.

Evidence from Sentry issue:

• 227 occurrences, 106 users affected (first seen 2026-07-07)
• github_event_action: stacked tag visible on events
• File: sentry/scm/private/webhooks/github.py, line 201
• Note: sentry-scm==0.32.1 (already released in the scm-platform repo) adds "stacked" to PullRequestAction, but the wheel is not yet available in the internal PyPI.

Fix: Catch msgspec.ValidationError in deserialize_github_pull_request_event and re-raise as SCMProviderEventNotSupported. This causes unknown PR actions to be silently counted as event-not-supported (the correct behavior for unrecognized GitHub event variants) rather than being reported as Sentry errors.

# Before
def deserialize_github_pull_request_event(event: SubscriptionEvent) -> PullRequestEvent:
    e = pull_request_decoder.decode(event["event"])

# After
def deserialize_github_pull_request_event(event: SubscriptionEvent) -> PullRequestEvent:
    try:
        e = pull_request_decoder.decode(event["event"])
    except msgspec.ValidationError as exc:
        raise SCMProviderEventNotSupported(
            f"Unsupported GitHub pull_request event: {exc}"
        ) from exc

A test test_produce_to_scm_stream_unknown_pr_action_is_not_reported is added to verify unknown actions are silently dropped without calling report_error.

Follow-up: Once sentry-scm==0.32.1 is available in the internal PyPI, bump the pin in pyproject.toml to get "stacked" into the type system properly.

Claude session: https://claude.ai/code/session_015pwRdeFeVj3ZSak5yEbtaY

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