minor: set default pooling back to retaining

[kgyrtkirk]

It turned out that the adaptive pooling (added in #20273) may help other issues to surface ; like #20522 (more detail about that is in #20475).

it would probably be safer to switch it off before it gets shipped in a release until we know its safe to turn it back on by default.

[FrankChen021]

🟢 Approval recommended

No actionable issues found in this review. Both HTTP client configuration entry points now default to retaining pooling, and the initialization path passes that selection explicitly to the resource pool. The documentation and configuration tests match the new default while preserving the explicit adaptive override.

Reviewed 4 of 4 changed files, with additional inspection of HTTP client wiring, pool lifecycle and concurrency behavior, and existing pool tests. Assessment is based on static inspection; tests were not run.

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This is an automated review by Codex GPT-5.6-Luna(max)