fix(truapi-server): raise the recursion limit for the request_login future

[decrypto21]

What

Adds #![recursion_limit = "256"] to truapi-server. Closes #476.

cargo +nightly clippy fails on all branches since 22 Aug. A recent nightly added recursion_depth_exceeding_limit; -D warnings promotes it to an error.

error: overflow evaluating the requirement ... pairing_host.rs:2423
  = help: consider increasing the recursion limit by adding a
          `#![recursion_limit = "256"]` attribute to your crate (`truapi_server`)
  = note: `-D recursion-depth-exceeding-limit` implied by `-D warnings`
  = warning: this was previously accepted by the compiler but is being phased out

Proving request_login's boxed trait future Send recurses through SsoPairingFlow::request_session, exceeding the default 128-step budget.

Verification

• 3 CI runs repo-wide since 22 Aug; all 3 failed (RFC 0027: Calling a product worker from its app #468, feat(truapi-server): host-side wire debug tap and debugger #295, docs(design): wire observability and debug host #315) with this error at pairing_host.rs:2423.
• docs(design): wire observability and debug host #315 is one markdown file and fails identically, so main is affected.
• Green Rust workspace elsewhere is from runs of 04–21 Aug, predating the lint.
• Local: fmt, clippy --workspace --all-targets --all-features -- -D warnings, test --workspace (23 suites, 1019 tests) pass.
• Local nightly is clippy 0.1.95 (Jan 2026), predating the lint; it passes either way. CI here is authoritative.

[decrypto21]

Closed because of #475.