Add list_code_scanning_alerts query-bounding guardrail and regression tests

[Copilot]

Unbounded list_code_scanning_alerts MCP calls return oversized payloads that break downstream workflow runs. Both GitHub MCP tools prompt files now instruct agents to always include state: open and severity: critical,high.

Changes

• actions/setup/md/github_mcp_tools_prompt.md — added **Code scanning queries** guardrail line requiring state: open and severity: critical,high on every list_code_scanning_alerts call
• actions/setup/md/github_mcp_tools_with_safeoutputs_prompt.md — same guardrail in the safe-outputs variant
• pkg/workflow/prompt_validation_test.go — three new regression tests:
  • TestGitHubMCPToolsPromptBoundsCodeScanningAlertQueries — asserts the plain prompt file contains both required parameters
  • TestGitHubMCPToolsWithSafeOutputsPromptBoundsCodeScanningAlertQueries — same assertion on the with-safeoutputs variant
  • TestGitHubMCPToolsPromptIncludedForCodeSecurityToolset — compiles a code_security-toolset workflow and verifies the generated lock file references one of the guardrail-carrying prompt files

The guardrail text added to both prompt files:

**Code scanning queries**: When calling `list_code_scanning_alerts`, always include
`state: open` and `severity: critical,high` to bound the response size and avoid
oversized payloads.

[pelikhan]

@copilot use the field selection feature from the GitHub MCP server 1.6.0 to apply these APIs. Update prompt.

[Copilot]

@copilot use the field selection feature from the GitHub MCP server 1.6.0 to apply these APIs. Update prompt.

Done in commit feat: add fields_param guidance to GitHub MCP tools prompts (MCP server 1.6.0).

Both prompt files now include a Field selection guardrail documenting the fields parameter introduced in GitHub MCP server 1.6.0 (available via the fields_param feature flag, auto-enabled in Insiders mode):

**Field selection (GitHub MCP server ≥ 1.6.0)**: When calling list/search tools that accept a `fields` parameter
(`list_pull_requests`, `list_issues`, `search_issues`, `search_pull_requests`, `list_commits`, `list_releases`,
`search_code`, `get_file_contents`), pass only the fields you need to reduce response size.
Example: `list_pull_requests` with `fields: [number, title, state, html_url]`.

Note: list_code_scanning_alerts does not have a fields parameter in v1.6.0 (not in the fields_param insiders feature set), so that tool continues to use the state: open / severity: critical,high filtering guardrail.

The SKILL.md Pull Requests section is also updated to prefer the fields approach over minimal_output for servers ≥ 1.6.0. A new regression test TestGitHubMCPToolsPromptHasFieldSelectionGuidance verifies the guidance is present in both prompt files.