perf(http): reuse one reqwest Client instead of building one per request

[allamiro]

Fixes #285.

Problem

Http::get_request built a fresh reqwest::Client (its own connection pool and TLS state) for every single request, so nothing was ever reused.

Change

The client is created once in Http::new and stored in the struct; get_request borrows it. Client::new() panics only on TLS backend initialization failure — same failure mode as before, just at construction.

Testing

cargo test: 9/9 pass (the mockito-based HTTP backend tests cover read/write paths).

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Summary by cubic

Reuse a single reqwest::Client across all HTTP calls to enable connection pooling and TLS session reuse, reducing per-request overhead. The client is now created once in Http::new and reused by get_request (same TLS init failure behavior, now at construction).

^{Written for commit 2744614. Summary will update on new commits.}