mcp: match multi-part starlark extensions in repository scanner

[a-palamarchuk]

What

The MCP repository scanner (used by mcp-server, mcp-server lint and dry-run)
lists .star.py and starlark as Starlark resource extensions, but matches them
against filepath.Ext. That function returns only the final extension (.py for
tool.star.py) and always includes the leading dot, so neither entry could ever
match. Files using those extensions were silently skipped and only .star files
were loaded.

This switches the matcher to suffix comparison and corrects starlark to
.starlark.

Reproduce (before this change)

A repository with resources/a.star, resources/b.star.py and
resources/c.starlark, each defining one mcp_tool(...):

$ redpanda-connect mcp-server ./repo   # tools/list
["from_star"]                           # b.star.py and c.starlark ignored, no warning

After: ["from_star", "from_star_py", "from_starlark"], and both new tools execute.

Tests

• New TestScannerStarlarkExtensions (fails on main, passes here). It also covers
  negative cases so the suffix match can't over-reach: a plain plain.py, a file
  named notstarlark, and starlark.txt are still ignored.
• Existing TestScannerHappy / TestScannerRoot still pass, covering .yml/.yaml
  matching for templates, resources and o11y config.
• go test -race -shuffle=on ./internal/mcp/..., the in-process MCP integration
  tests (-run '^TestIntegrationMCP'), and ./internal/cli/ all pass.
  golangci-lint run and golangci-lint fmt --diff are clean.

Notes

• Behaviour change: .star.py / .starlark files that were previously ignored in
  resources/ will now be evaluated, so an invalid one would now fail startup the
  same way an invalid .star file already does.
• Out of scope: dry-run returns "resource type 'starlark' is not supported yet"
  for Starlark files. That was already true for .star; this change only makes the
  other two extensions behave consistently with it.

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