[test] Add tests for launcher.GetOrLaunchForSession

[github-actions]

Test Coverage Improvement: GetOrLaunchForSession

Function Analyzed

• Package: internal/launcher
• Function: GetOrLaunchForSession
• File: internal/launcher/launcher.go:180
• Previous Coverage: 22.9%
• New Coverage: 79.2%
• Package Coverage: 84.4% → 93.0%
• Complexity: High (multiple branches: unknown server, HTTP backend, session pool cache hit, double-check lock, container warning, stdio launch)

Why This Function?

GetOrLaunchForSession had the lowest coverage (22.9%) among complex functions in the codebase. It manages session-scoped MCP backend connections with a double-check locking pattern, security warnings for containerized environments, and multiple backend types. Despite 18 existing tests, all were skipped because they assumed echo implements the MCP protocol handshake. Only the HTTP backend and invalid-server tests provided any coverage.

Tests Added

• ✅ Session pool cache hit (TestGetOrLaunchForSession_StdioSessionPoolHit): Pre-populates the session pool using a real HTTP connection pointer, verifies cache hit returns the existing connection without re-launching
• ✅ Launch failure error handling (TestGetOrLaunchForSession_StdioLaunchFailure): Uses a nonexistent command to verify error propagation from the stdio launch path
• ✅ Container security warning path (TestGetOrLaunchForSession_DirectCommandWarningInContainer): Sets runningInContainer=true to exercise the security warning branch for direct (non-docker) commands
• ✅ Multiple session cache hits (TestGetOrLaunchForSession_StdioSessionPoolHit_DifferentSessions): Verifies independent session IDs each resolve their own cached connections

Coverage Report

Before: 22.9% coverage  (GetOrLaunchForSession)
After:  79.2% coverage  (GetOrLaunchForSession)
Improvement: +56.3%

Package overall:
Before: 84.4%
After:  93.0%
````

## Test Execution

All new tests pass:
````
ok  	github.com/github/gh-aw-mcpg/internal/launcher	17.090s	coverage: 93.0% of statements

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[Copilot]

Pull request overview

This PR adds comprehensive test coverage for the GetOrLaunchForSession function in the launcher package, improving coverage from 22.9% to 79.2% and bringing overall package coverage to 93.0%. The function manages session-scoped MCP backend connections with complex logic including double-check locking, session pool caching, and container security warnings.

Changes:

• Added 4 new test cases targeting previously uncovered code paths (session pool cache hits, launch failures, container warnings, multiple sessions)
• Introduced newMockHTTPMCPServer helper function to support testing session pool behavior without requiring actual MCP protocol implementations

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