gateway: harden and simplify MCP gateway code

[dgageot]

Summary

Harden and simplify the MCP gateway code across pkg/gateway and pkg/tools/mcp.

Changes

• Remove loadCatalog indirection — it accepted a context but ignored it, just forwarding to catalogOnce(). Call catalogOnce() directly.
• Pass secrets directly to NewGatewayToolset — eliminates a redundant ServerSpec round-trip; the caller already has the server spec.
• Fix temp file leak in writeTempFile — on write or close error, the temp file is now removed. Close errors are no longer silently dropped.
• Log cleanup warnings scoped to temp file errors — GatewayToolset.Stop previously logged warnings for routine process-exit errors too.
• Reject secret values containing newlines — prevents injection in the line-based secrets file format.
• Use dedicated http.Client — isolates catalog fetching from any mutations to http.DefaultClient.
• Lazy MkdirAll in saveToDisk — try CreateTemp first; only create the directory on ENOENT.
• Merge servers.go into catalog.go — ParseServerRef was the only function; reduces file count.
• Explicit catalogJSON constant — replaces fragile strings.Replace derivation.
• Network-independent tests — inject a self-contained test catalog via TestMain instead of hitting the live Docker MCP catalog.

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

This PR successfully hardens and simplifies the MCP gateway code with several important improvements:

✅ Security improvements: Newline validation prevents injection attacks in secrets files
✅ Resource management: Fixed temp file cleanup on write/close errors in writeTempFile
✅ Code simplification: Removed unnecessary indirection and merged related code
✅ Test reliability: Network-independent tests via injected catalog
✅ Better error handling: Explicit logging of cleanup warnings in Stop()

Minor Observation

While reviewing the temp file lifecycle in NewGatewayToolset, I noticed that if initialization fails after writeConfigToFile succeeds (e.g., if NewToolsetCommand fails), the fileConfig temp file cleanup depends on the caller invoking Stop() on the returned GatewayToolset. If callers don't call Stop() on initialization failures, temp files could accumulate in long-running processes.

Suggestion: Consider documenting that callers must call Stop() even on initialization failures, or use defer patterns to ensure cleanup happens automatically.

This is a minor edge case and doesn't block the PR. The changes are solid improvements to the codebase.

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