Return a clear error for missing owner/repo/issue_number in the copilot assignment tools

[thejdubb02]

Summary

Return a clear missing required parameter error when owner, repo, or issue_number is omitted from assign_copilot_to_issue / assign_copilot_to_issue_with_intent, instead of a confusing downstream repository error.

Why

Both handlers decode these required params with mapstructure.WeakDecode, which zero-fills a missing value instead of erroring. A missing arg then reached the GraphQL query and surfaced as failed to get suggested actors: ... Could not resolve to a Repository, which does not tell the caller what was actually wrong. The input schema already marks these three fields required, so this makes the handler enforce it, matching the existing is_suggestion and rationale/confidence checks in the same handlers and the RequiredParam usage in request_copilot_review.

What changed

• After decoding, reject a zero-value owner, repo, or issue_number with missing required parameter: <name> in both AssignCopilotToIssue and AssignCopilotToIssueWithIntent.
• Added table tests for the missing cases.

MCP impact

• Tool schema or behavior changed
  • No schema change (the three fields were already marked required). The two tools now return a clear missing required parameter error for a missing required field instead of a confusing repository-resolution error.

Prompts tested (tool changes only)

• "Assign Copilot to this issue" with owner omitted now returns missing required parameter: owner. Verified via the new unit tests in copilot_test.go: a request missing each required field asserts the clear error, and without the fix it returns the Could not resolve to a Repository 404 instead.

Security / limits

• No security or limits impact

Tool renaming

• I am not renaming tools as part of this PR