Stabilize Session protocol full-suite tests

[yyjeqhc]

Summary

• run the large request-scoped ACK HTTP integration fixture on an explicit 8 MiB test thread stack without changing production request handling
• align Session collaboration OAuth/shared-key test fixtures with the new session:collaborate authority split
• keep startup workflow output schema and compact model guidance synchronized with piggyback Session message resolution
• make the Windows LSP initialize-cleanup budget test assert the supervisor-owned restart outcome rather than a scheduler-sensitive child start marker; disable its unrelated background reaper

Validation

• cargo fmt -- --check
• focused startup brief tests: 14 passed
• focused collaboration tests: 18 passed
• focused request-scoped ACK HTTP test: 1 passed
• focused Session OAuth scope test: 1 passed
• full cargo test on the implementation commit: 3020 passed, 0 failed
• follow-up LSP initialize-cleanup regression on the current test-only head: 20/20 focused runs passed
• git diff --check