chore(http): Enable pedantic Clippy lints

[pront]

Summary

Motivation

Continue the incremental pedantic Clippy rollout in shared HTTP helpers and both HTTP client implementations.

Changes

Enable clippy::pedantic for the HTTP module, add must_use annotations, and apply mechanical statement and rustdoc fixes. Keep deferred cleanup in narrow allowances with concise reasons linked to #23659. Use the standard OAuth 2.0 name in bearer-token documentation.

References

Related: #23659. Split from #26574.

How did you test this PR?

All 39 HTTP tests passed, including legacy and v1 client coverage for TLS, mutual TLS, authenticated proxies, credential isolation, and proxy bypass. Workspace/all-target Clippy passed with api,aws-core; formatting, documentation generation, and whitespace checks passed.

Does this PR include user facing changes?

• Yes.
• No.

[datadoghq-integration]

More details

The added semicolons preserve unit-returning authentication behavior, while targeted lint allowances retain existing ownership, drop timing, and return types.

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[datadoghq-integration]

More details

The semicolon edits preserve unit-valued behavior; the remaining changes affect lint annotations and synchronized documentation without changing HTTP transport logic.

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