Backfill changelogs for 2.6.0-2.8.0 and cut 2.9.0

[Kludex]

Summary

Releases v2.6.0, v2.7.0, and v2.8.0 were tagged without changelog entries. This backfills both package changelogs from the commit history between tags, and renames Unreleased to 2.9.0.

• httpx2: 2.6.0 (WebSocket support via Add native WebSocket support by vendoring httpx-ws #1042, QUERY method via Add QUERY method support (RFC 10008) #1055, click bounds via Bump click lower bound to >=8.4 and drop upper bound #1040), 2.7.0 (httpx-ws v0.9.0 via Update vendored httpx-ws to upstream v0.9.0 #1067), 2.8.0 (lockstep bump, no changes), 2.9.0 (alias_httpx() via Add alias_httpx() to alias httpx imports to httpx2 #1077, with the PR link added).
• httpcore2: 2.6.0 (fix: clean up garbage connections on cancellation in AsyncConnectionPool #983, Read _expire_at only once in has_expired() #1045), 2.7.0 (lockstep bump), 2.8.0 (connection pool churn via Eliminate multi-assignment churn in the connection pool #1075), 2.9.0 (lockstep bump).

Docs-only and test-only commits (#1041, #1054, #1058, #1061, #1062, #1063, #1065) are omitted, matching existing changelog conventions. Lockstep entries reuse the phrasing from httpcore2 2.2.0. Dates come from the tag dates; 2.9.0 is dated today.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview: https://228a5f1e-httpx2-docs.pydantic.workers.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing backfill-changelogs (b509f0d) with main (51d8234)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[chatgpt-codex-connector]

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