test: drop misleading async on sync streaming tests + lock random.seed guard (#2071)

[yastman]

This pull request was created by @kiro-agent on behalf of @yastman 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Parent: #1515 (test audit), category S2 + S3 (code smells).

This PR addresses two of the four #2071 cleanup categories incrementally and behavior-preservingly:

S2 — async def without await

tests/unit/agents/test_streaming.py declared four async def test_* that never awaited. pytest-asyncio mode=auto silently ran them in an event loop. Stripped async from those four tests. Behavior is identical: pytest collects and runs them as plain sync functions, the same assertions hold.

S3 — random.seed in test bodies

Added tests/contract/test_no_global_random_seed_contract.py. Counts random.seed(...) vs random.getstate() per file under tests/ and fails if seed > getstate (i.e. someone introduces a bare random.seed without a save/restore guard).

Current state on dev: tests/unit/scripts/test_kommo_seed.py has 3 seed calls, all wrapped in random.getstate()/setstate() try/finally blocks. The contract passes today and prevents regression.

What this PR explicitly does NOT touch

• Long test names (S5) — left alone; renaming would break --lf/--ff pytest history with no behavior win.
• uuid4 usage (S6) — issue test: CODE-SMELLS cleanup async/no-await, random seed, timing, and names (#1515) #2071 itself says "Leave uuid4 usage alone unless it causes parallel-test instability."
• Timing assertions (S4) — no concrete regressions surfaced; defer to a follow-up scoped at the specific timing-flaky tests.

Verification

uv run pytest tests/unit/agents/test_streaming.py \
              tests/contract/test_no_global_random_seed_contract.py -q
  5 passed

uvx ruff check tests/unit/agents/test_streaming.py \
               tests/contract/test_no_global_random_seed_contract.py
  All checks passed!

Closes #2071.