feat(waterdata): Migrate to httpx and add async parallel chunker

[thodson-usgs]

Summary

Stacked on top of the chunker-unified work (PR #283); rebase to main once that lands.

• Replaces requests with httpx package-wide.
• Adds an opt-in async parallel branch to the multi-value chunker via API_USGS_CONCURRENT (default 16 for the server-friendly sweet spot; set =1 for the legacy sequential path).
• Integrates the async path with the ChunkPlan / ChunkedCall arch from feat(waterdata): Auto-chunk OGC requests over the URL byte limit #283 — sync drives ChunkedCall.resume() over one shared httpx.Client; parallel uses _fan_out_async to iterate the same plan via asyncio.gather + asyncio.Semaphore over one shared httpx.AsyncClient.

Benchmarked at 2.48× speedup on a 19,602-site / 6-state get_daily call at API_USGS_CONCURRENT=16 (2.78s vs 6.89s sequential, distinct date windows per trial so cache reuse can't bias the result).

Why httpx

httpx ships sync and async clients on a unified API, so the same request shape powers both the existing synchronous getters and the new parallel path. requests is unmaintained and has no async story — a thread-pool bolt-on would have been a one-off rather than a primitive reusable elsewhere.

API_USGS_CONCURRENT

Value	Behavior
unset / blank	parallel, cap = 16 (_CONCURRENCY_DEFAULT)
≥ 2	parallel, semaphore-capped at that value
1	serial (sync ChunkedCall.resume() path)
unbounded	parallel, no per-call cap — caller owns the burst risk
0, negative, malformed	ValueError at call time

Connection-pool sharing across all sub-requests of a single chunked call in both modes via the _chunked_session (sync) / _chunked_async_session (async) ContextVars — _walk_pages / _walk_pages_async / get_stats_data read them as fallbacks before opening a fresh client.

Parallel path safety contracts

The parallel branch preserves the same safety contracts the serial path provides:

• Probe-first quota check. _fan_out_async issues the first sub-request alone, reads x-ratelimit-remaining from its response, and raises RequestExceedsQuota before dispatching the rest if the remaining plan can't fit the window. Matches ChunkedCall._check_quota_after_first.
• Resumable interruptions. asyncio.gather runs with return_exceptions=True, so a sibling's transient failure (RateLimited / ServiceUnavailable) doesn't lose the completed work. The raised ChunkInterrupted.call is a ChunkedCall holding the sparse-indexed completed sub-requests; exc.call.resume() re-issues only the unfinished indices via the sync fetch_once path.
• Event-loop detection. asyncio.run() raises inside an already-running loop (Jupyter / IPython kernels, async apps). The wrapper calls asyncio.get_running_loop() first and, when one is active, falls back to the serial path with a UserWarning instead of crashing.
• Missing-fetch_async warning. If API_USGS_CONCURRENT requests parallel but the decorator wasn't wired with fetch_async=, the wrapper warns + runs serial rather than silently no-op'ing the env var.

Three httpx behavior diffs handled defensively

• httpx.InvalidURL raised when a URL component > 64 KB (e.g. all California stream sites comma-joined in one query). Caught by _safe_request_bytes (treats "too big to construct" as "doesn't fit", so the planner's halving loop keeps shrinking) and again in ChunkPlan.__init__ so canonical-URL recovery can fall through to a worst-case sub-request URL.
• httpx.Response.elapsed only populated on close (not by httpx.MockTransport / pytest-httpx). _safe_elapsed falls back to timedelta(0).
• httpx.Response.url is a read-only property. _set_response_url rewrites it via the bound request, with a fallback path for Mock-shaped test responses.

Backwards-compat

• BaseMetadata.header is now httpx.Headers instead of requests.structures.CaseInsensitiveDict. Case-insensitive lookups (md.header.get("x-ratelimit-remaining")) keep working; literal dict equality (md.header == {"k": "v"}) no longer holds because httpx.Headers carries auto-added entries (content-type, content-length).
• BaseMetadata.url is coerced to str (previously str on requests.Response; now str(httpx.Response.url)).
• API_USGS_CONCURRENT defaults to 16 (parallel). Set =1 to opt back into the sequential path.

Test plan

• 367 mocked tests pass after migrating to pytest-httpx. tests/conftest.py (new) is a requests_mock-shaped shim over httpx_mock (URL-prefix match, complete_qs strict-mode parity, request-history view); an autouse fixture pins API_USGS_CONCURRENT=1 so the historical mocked suite stays on the deterministic serial path.
• Async-path coverage: four new async-mode test functions (one parametrized over running-loop and missing-async) in tests/waterdata_chunking_test.py opt out of the serial-pin autouse and exercise (a) successful fan-out, (b) probe-first RequestExceedsQuota, (c) resumable ServiceInterrupted.call after a mid-fan-out failure (resume runs serially on the unfinished indices), (d) the running-event-loop fallback emits a UserWarning and runs serial, (e) the missing-fetch_async warning fires when the env asks for parallel.
• ruff check and ruff format --check pass.
• Live-API CI sweep — most pre-existing column-drift failures (get_daily / get_stats_* / get_channel) are resolved by the schema-aware _get_resp_data / _handle_stats_nesting rewrite incorporated via the rebase.

Out of scope (follow-ups)

• High-concurrency memory: _fan_out_async materializes all (df, response) pairs before combining. Consider streaming-combine via asyncio.as_completed if users push concurrency very high.
• NEWS.md entry — left for the merger to draft.

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