[test-improver] Improve tests for server session package

[github-actions]

File Analyzed

• Test File: internal/server/session_test.go
• Package: internal/server
• Lines Added: 208 (137 → 345)

Improvements Made

1. Increased Coverage

Four methods in session.go had zero test coverage. This PR adds tests for all of them:

Method	Before	After
getSessionID	❌ Not tested	✅ 5 table-driven cases
ensureSessionDirectory	❌ Not tested	✅ 3 sub-cases
requireSession	❌ Not tested	✅ 4 sub-cases incl. concurrency
getSessionKeys	❌ Not tested	✅ 3 sub-cases

2. Better Testing Patterns

• ✅ Added newMinimalUnifiedServer helper with t.Helper() and t.Cleanup() to reduce boilerplate across tests
• ✅ Used require.NoError for critical setup steps, assert for non-fatal checks
• ✅ Proper subtests via t.Run() with descriptive names
• ✅ t.TempDir() for filesystem isolation — no leftover state between runs

3. Cleaner & More Stable Tests

• ✅ Each subtest creates its own UnifiedServer (or resets payloadDir) so tests are fully independent
• ✅ Concurrency test for requireSession verifies the double-checked locking ensures sessions are created exactly once under load
• ✅ assert.Same used to verify pointer identity (not just value equality) for session reuse

Key New Tests

TestGetSessionID — confirms getSessionID is a transparent wrapper for SessionIDFromContext across absent, empty, non-string, and valid inputs.

TestEnsureSessionDirectory — confirms the function creates payloadDir/sessionID/, is idempotent, and handles multiple sessions independently.

TestRequireSession — confirms sessions are auto-created on first call, reused on subsequent calls, and that 20 concurrent goroutines all see the same single session instance.

TestGetSessionKeys — confirms an empty server returns no keys, and that keys match exactly the set of sessions created via requireSession.

Why This File?

session.go implements the session lifecycle for the gateway (creation, directory management, lookup). The existing session_test.go only tested the two pure functions (SessionIDFromContext, NewSession) and left the four UnifiedServer methods completely uncovered. These methods contain non-trivial logic: directory creation, syncutil.GetOrCreate double-checked locking, and map iteration.

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