Seer Autofix called for metric issues but not supported

[Dhrumil-Sentry]

When an agent attempts to root-cause a metric_issue type issue, it calls the Seer Autofix/RCA tool. Seer Autofix does not support metric issues, so the call fails or returns an unhelpful result.

• Issue type affected: metric_issue / category: metric
• Observed behavior: agent calls Seer RCA tool on a metric alert issue
• Expected behavior: agent skips Seer Autofix for metric issues (or the tool returns a clear "not supported" signal), and falls back to a strategy that is actually useful (e.g. fetching the alert rule query and searching for related error issues)

The tool should either:

1. Guard against calling Seer for metric_issue type issues, returning a clear unsupported message, or
2. Document the limitation so the agent can route around it

[linear-code]

MCP-983