feat: add HTTP client connection

[algebraic-dev]

This PR adds a Connection module that bridges the async runtime with the H1 state machine with with a Session structure that manages the lifecycle of a single Connection handling both outgoing requests and incoming responses.

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

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

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• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto 110db9cb751afaee8b2ac344887d6c7e632f77b4. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-04 13:16:47)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto 945e78b86645b179655123cd2fdba83d89d28d07. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-04 20:36:46)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto f2bcf2e8660ab2d16cf3cb50c8e127de0439a337. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-05 16:31:14)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto bd97ed8e35639e17a11d1490cbc46d994e7b4513. You can force Mathlib CI using the force-mathlib-ci label. (2026-08-07 16:43:41)

[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 14:51:09)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto 23393b959b33e3a8d15796b2397f8a04c315b9f4. You can force reference manual CI using the force-manual-ci label. (2026-08-04 13:16:48)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 6832de296014011d0f126e3fa32a8366b5299b35 --onto c4e6b62c3d955ef20da94310797072f7c4c5fa2b. You can force reference manual CI using the force-manual-ci label. (2026-08-07 16:43:44)

[TwoFX]

Just some preliminary comments, I will have to read through the logic in more detail in August.