Add metric alert context to Seer issue details RPC

[JoshFerge]

Summary

• Add optional metric_alert_context to Seer's Sentry RPC issue-details payload for metric issues.
• Extract the context from serialized occurrence data so downstream agents can see alert aggregate, query, conditions, observed value, and detector/alert IDs.
• Cover the new extraction and forwarding behavior in Seer agent tool tests.

Companion PR

• Seer consumer/formatter PR: https://github.com/getsentry/seer/pull/6360

Tests

• .venv/bin/pytest -n0 -svv --reuse-db tests/sentry/seer/agent/test_tools.py -k "TestGetIssueDetails or TestGetIssueAndEventDetailsV2 or extract_metric_alert_context"
• .venv/bin/prek run -q --files src/sentry/seer/agent/tools.py tests/sentry/seer/agent/test_tools.py
• git diff --check