test(core): simplify runner test fixtures

[kitlangton]

Summary

• replace repeated LLM event envelopes with small pure response helpers
• collapse default prompt admissions into a focused admit helper
• centralize runner fixture state reset and return the Session service from setup
• preserve explicit raw streams and race controls where event ordering matters

Result

• session-runner.test.ts: 4,345 to 3,711 lines
• 634 net lines removed
• no new fixture framework or test files
• assertion count preserved

Verification

• bun run test test/session-runner.test.ts test/session-runner-recorded.test.ts (106 passed)
• bun run test test/session-runner.test.ts --randomize --rerun-each 5 (525 passed)
• bun typecheck in packages/core
• pre-push workspace typecheck (31 packages passed)