fix(tracing): never let span export block the event loop or the run path

[xfgong]

Problem

Diagnosed in cubebox: every chat turn's `done` event lagged the final `usage`
event by 5–20s. Timing probes put 8.65s inside the exit of
`async with trace(tracer, agent)` — span export was gating a user-visible
response, and worse, freezing the whole event loop while it ran:

1. `Tracer.force_flush` (and `Meter.force_flush`) are `async def` but called
   the provider's synchronous `force_flush` inline. Awaiting them parked a
   blocking flush on the loop thread: every concurrent request in the host
   process stalls until the exporters drain. Against a remote/backlogged OTLP
   collector that's seconds per run.
2. `trace()` awaits the flush task on exit by design ("exit means exported").
   Right default for scripts; wrong for serving paths where a caller is
   blocked on the `async with` body's completion.

Fix

• `Tracer.force_flush` / `Meter.force_flush` now run the provider flush via
  `asyncio.to_thread` — awaiting them never stalls the loop.
• `trace()` gains `flush: Literal["await", "background"] = "await"`.
  • `"await"` (default): unchanged semantics.
  • `"background"`: detach stays synchronous (listeners off on the next
    line), export runs as a supervised background task. The tracer holds
    strong refs (loop refs are weak) and `shutdown()` settles pending
    flushes, so spans still export on clean shutdown. Supervisor logs
    failures, matching `trace()`'s swallow-and-log contract.

Tests

`tests/tracing/test_nonblocking_flush.py`:

• provider flush runs off the loop thread; a 0.5s export no longer freezes a
  concurrently ticking task
• `flush="background"` exits the block immediately, tracks the task, and
  `shutdown()` settles it; failures are logged
• regression pin: default mode still blocks until export completes

Full suite: 1842 passed, 1 skipped. ruff + format + mypy clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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