fix(libsy-llm-client): bound upstream requests with a configurable timeout

[gburachas]

Problem

The Rust server's HTTP client is built with no request timeout:

reqwest::Client::builder().build()

reqwest applies no overall timeout unless told to, and there is no configuration key to set one.
A request to a hung or unresponsive upstream therefore waits indefinitely — the connection is
held, the caller never gets an answer, and an operator has no way to bound it.

The retry budget does not help: retries fire on failures, and a request that never returns never
fails.

The capability exists elsewhere in the workspace — switchyard-components (the Python-facing path)
takes timeout_secs: Option<f64>, validates it, and passes it to the same reqwest builder. The
Rust server path simply never wires it up. The crate's own tests set .timeout(...) directly, so
the mechanism works; only the configuration is missing.

Proposal

• timeout_secs: Option<f64> on the client's backend config, exposed as timeout_secs under
  [llm_clients.<name>];
• applied per attempt via RequestBuilder::timeout, so each retry gets its own budget;
• validated finite and positive at load time;
• unset by default, preserving today's behaviour exactly.

Deliberately reuses the seconds-as-f64 shape and the validation wording already used by
switchyard-components, rather than introducing a second convention.

Documented caveat

Because the bound is per attempt, a call that exhausts its retry budget can take up to
(max_retries + 1) x timeout_secs. Stated in the schema reference next to the key.

Scope

6 files, +83/−2. Core change is ~15 lines; the rest is config plumbing, two struct literals, one
test, and the docs row.

Validation

Gate	Result
cargo test --workspace	641 passed (baseline 640 + 1)
cargo clippy --workspace --all-targets -- -D warnings	clean
cargo fmt --check	clean
uv run pytest tests/	unchanged

The new test drives a mock upstream that delays 30s under a 50 ms timeout with retries disabled, and
asserts the call fails as Timeout rather than hanging.