fix: handle PII-scrubbed span values and unknown GitHub PR actions gracefully

[billyvg]

Fixes two escalating Sentry alerts surfaced by automated monitoring.

SENTRY-5R9R — ValueError in LargeHTTPPayloadDetector (HIGH actionability, 1256 occurrences)

Root cause: large_http_payload_detector.py:59 calls int(encoded_body_size) where encoded_body_size is a string. When Sentry's PII data scrubber replaces the value with '[Filtered]', the int() conversion throws ValueError, which is caught upstream as "Failed to detect performance problems" — silently skipping performance issue detection for that span.

Fix: Wrap the int() call in try/except ValueError and return early, so scrubbed spans are skipped cleanly.

Evidence: Stacktrace shows encoded_body_size = "'[Filtered]'" as a local variable at the crash point. Issue link: https://sentry.sentry.io/issues/SENTRY-5R9R

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SENTRY-5R97 — ValidationError: Invalid enum value 'stacked' in GitHub webhook (MEDIUM actionability, 177 occurrences, 93 users)

Root cause: GitHub introduced a new pull request action type "stacked" (stacked PRs feature). Our GitHubPullRequestEvent msgspec struct used PullRequestAction (a strict Literal[...] type) for the action field, so msgspec's decoder throws ValidationError on any action not in the enum. This generates a Sentry error for every stacked PR event from any customer's GitHub integration.

Fix:

• Widen GitHubPullRequestEvent.action from PullRequestAction to str so msgspec accepts any string without failing validation.
• Compute _KNOWN_PR_ACTIONS at import time from typing.get_args on the PullRequestAction type alias (so it stays in sync when sentry-scm is bumped).
• In deserialize_github_pull_request_event, return None for unrecognised action types with an info-level log instead of crashing. The caller (deserialize_github_event) already returns EventType | None, so None events are silently dropped.

Note: sentry-scm==0.32.1 (released 2026-07-10) already adds "stacked" to PullRequestAction, but it hasn't been published to the internal PyPI yet. This sentry-side fix provides immediate relief and future-proofs us against any other new GitHub action types.

Evidence: Error tags show github_event_action: stacked. Issue link: https://sentry.sentry.io/issues/SENTRY-5R97

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Changes

• src/sentry/issue_detection/detectors/large_http_payload_detector.py: try/except ValueError around int(encoded_body_size)
• src/sentry/scm/private/webhooks/github.py: widen action field to str, validate against _KNOWN_PR_ACTIONS, return None for unknown actions

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Claude session: https://claude.ai/code/session_015Gd856MCZWtWz8VmqLdaA3

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