feat: add HTTP client Pool

[algebraic-dev]

This PR adds a Pool structure that manages multiple open connections, allowing them to be reused across requests.

• Foundation: feat: add HTTP client foundation and configuration #14546
• Connection: feat: add HTTP client connection and session #14547
• Agent: feat: add HTTP client Agent #14548
• Pool: feat: add HTTP client Pool #14549

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-27 16:01:36)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto 0bfc3acaef4ed0576307a77fbaa0c6e1a5dca402. You can force reference manual CI using the force-manual-ci label. (2026-07-27 16:01:38)