refactor: convert sendAIMessage from recursion to iterative while-loop

[lewis617]

Summary

Convert the recursive sendAIMessage method in aiManager.ts to an iterative nested while-loop design, eliminating stack growth that caused OOM at depth 2418+.

Changes

• Nested while-loops: outer loop handles restarts (notifications, goal continuation, stop hooks); inner loop handles tool-call continuation
• turnOffset (= initial recursionDepth) gates depth-0 init and finally-equivalent cleanup — same semantics as the old recursionDepth === 0 checks
• turnDepth (= turnOffset + inner loop counter) replaces recursionDepth in maxTurns check
• Removed setImmediate hack for stack growth (PR fix: yield event loop before sendAIMessage recursion to prevent OOM #1325) — while-loop doesn't grow stack
• Kept setImmediate yield before inner continue — needed so macrotasks (abort timers) can fire between turns in mocked test scenarios where all async ops are microtasks
• try/catch/finally became try/catch inside inner loop + sequential finally-equivalent code after it
• All 4 recursive call sites converted:
  • Tool-call continuation → turnDepth++; continue inner;
  • Notification/goal/stop-hook restarts → shouldRestart=true; turnOffset=0
• Renamed agent.toolRecursion.test.ts → agent.toolLoop.test.ts

What stays the same

• Method signature unchanged (recursionDepth param kept for backward compat)
• All external callers unchanged
• Error handling, saveSession counts, OTel spans, loading state, maxTurns enforcement

Verification

• All targeted test files pass (85 tests)
• Full agent-sdk suite: 3151 passed, 1 skipped (235 files)
• Coverage tests (original OOM scenario): pass
• Type-check (all 3 packages): clean

Closes #1324