fix: add max_length support to Gzip/Brotli decoders for urllib3 2.6+

[CalNightingale]

Summary

Fixes #491. Supersedes #492 (stale).

urllib3 >= 2.6.0 now passes max_length to decoder.decompress() and accesses decoder.has_unconsumed_tail. Our custom _GzipDecoder and _BrotliDecoder wrappers didn't accept these, causing TypeError at runtime.

This PR:

• Updates _GzipDecoder.decompress and _BrotliDecoder.decompress to accept and forward max_length, with a TypeError fallback for older urllib3
• Adds has_unconsumed_tail property to _BrotliDecoder (the proxy class that needs it explicitly)
• Applies the same fix to the async _GzipDecoder in google/_async_resumable_media/
• Adds test coverage for the new max_length forwarding, the fallback path, and has_unconsumed_tail

Feedback from #492 addressed

• max_length=-1 default (chandra-siri) — uses -1 to match urllib3's default, not None
• has_unconsumed_tail only on _BrotliDecoder (chandra-siri) — _GzipDecoder inherits this from the parent urllib3.response.GzipDecoder, so no override is needed
• Lint failure (BrennaEpp) — no trailing whitespace
• Missing test coverage for fallback paths (BrennaEpp) — added tests for max_length forwarding, TypeError fallback, and has_unconsumed_tail (with AttributeError fallback for older urllib3)

Test plan

• pytest tests/unit/requests/test_download.py — all pass
• pytest tests_async/unit/requests/test_download.py — all pass
• Full unit suite (514 tests) — all pass
• No lint issues (flake8 clean)

🤖 Generated with Claude Code

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[chalmerlowe]

LGTM