feat(async): add HTTPClientConfig for tuning the async transport (closes #170)

[be-ant]

Summary

• Adds HTTPClientConfig — a Pydantic model with scalar fields only — that lets users tune the async transport without leaking httpx types into the public API.
• Wires it through AsyncOFSC(..., http_config=...); defaults preserve today's behavior (HTTP/2 on, default pool/timeout, no retries).
• Headline knob is max_concurrency, which translates to httpx.Limits(max_connections=N, max_keepalive_connections=N) — same semantics as aiohttp's TCPConnector(limit=N), so the abstraction survives a transport library swap.
• Also exposes: timeout, max_retries (transport-level), proxy, verify_ssl, http2, follow_redirects, trust_env.
• Adds 17 unit tests covering model validation and the wiring into httpx.AsyncClient.
• README gains a short "HTTP Transport Configuration" subsection.

Closes #170.

Why library-neutral

HTTPClientConfig deliberately exposes only int / float / bool / str. The httpx.Limits / httpx.Timeout / httpx.AsyncHTTPTransport translation lives entirely inside AsyncOFSC._build_client_kwargs. If the project ever swaps httpx for another async HTTP client, the public surface here stays the same.

Example

from ofsc.async_client import AsyncOFSC, HTTPClientConfig

async with AsyncOFSC(
    clientID="...", companyName="...", secret="...",
    http_config=HTTPClientConfig(
        max_concurrency=20,
        timeout=30.0,
        max_retries=2,
    ),
) as client:
    workzones = await client.metadata.get_workzones()

Backward compatibility

http_config defaults to None. When unset, the constructor builds the client exactly as before. No existing call sites need to change.

Test plan

• uv run pytest tests/async/test_async_http_config.py -v — 17 new tests pass
• uv run pytest tests/async/test_async_ofsc.py -v — existing async client tests still pass
• uv run pytest -m "not uses_real_data and not uses_local_data and not integration" — 443 passed, 8 skipped
• uv run ruff check clean on changed files
• uv run ruff format --check clean on changed files
• Reviewer: run uv run pytest with live credentials to confirm no regression in credentialed tests