chore(routers/http): remove dead code and trim stale comments

[slin1237]

Description

Problem: the HTTP/PD routers carry provably-dead internal code and stale comments flagged by the codebase audit (gw-routers-http) — an unused error enum, an unused URL helper, an unused bootstrap-wrapper struct, a dead function parameter, and comments that are either wrong or merely restate the code.

Solution: removed the dead internal items and trimmed the flagged comments. No behavior change.

From the codebase audit (gw-routers-http): dead_code + comment_bloat findings.

Changes

Dead code (4 internal items, all re-verified unused repo-wide incl. tests/benches/bindings/clients):

• pd_types.rs: removed unused PDRouterError enum (zero constructors anywhere), api_path helper (no callers), and BatchRequestWithBootstrap struct (no references). The smg crate is consumed only by the in-workspace bindings, which glob-import the crate root (use smg::*); the root does not re-export pd_types, so these pub items are unreachable as external API.
• pd_router.rs: dropped the unread _start_time: Instant parameter from execute_dual_dispatch_internal (and its now-unnecessary #[expect(clippy::too_many_arguments)], which became unfulfilled at 6 args).

Comments (4 lines, 3 sites):

• pd_router.rs get_server_info: deleted two stale comments claiming server info comes from the "decode server" — the code proxies to the first prefill worker.
• pd_router.rs inject_bootstrap_into_value: removed the two repeated "Use static string keys to avoid per-request allocations" comments that restate the code (and are partially inaccurate, since the Map insert still allocates an owned String key).

Kept (re-verified as live test/bench fixtures, not dead): RequestWithBootstrap (used by benches/request_processing.rs) and PDSelectionPolicy (used by tests/routing/test_pd_routing.rs).

Net: 51 deletions across 2 files (deletion-heavy cleanup; well under 400 lines).

Test Plan

Gate run with sccache disabled, scoped to the smg package (CI runs the full --all-features workspace gate):

cargo +nightly fmt --all — clean (pure deletions, no reformatting).

RUSTC_WRAPPER="" cargo clippy -p smg --all-targets -- -D warnings:

    Checking smg v1.4.1 (.../model_gateway)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1m 12s

(only pre-existing workspace notes: non-root profiles, duplicate main.rs build target — unrelated to this change)

RUSTC_WRAPPER="" cargo test -p smg:

test result: ok. 1003 passed; 0 failed; 4 ignored (lib)
api_tests: 105 passed; 0 failed
routing_tests: 92 passed; 0 failed
reliability_tests: 26 passed; 0 failed
security_tests: 48 passed; 0 failed
... all integration + doc suites: 0 failed

• cargo +nightly fmt --all
• cargo clippy --all-targets -- -D warnings

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 38307361-322b-41b2-b339-8cfe44d941a6

📥 Commits

Reviewing files that changed from the base of the PR and between 0405bf6 and 6b98cee.

📒 Files selected for processing (2)

• model_gateway/src/routers/http/pd_router.rs
• model_gateway/src/routers/http/pd_types.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/tidy-gw-routers-http

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[gemini-code-assist]

Code Review

This pull request cleans up unused code and parameters in the prefill-decode (PD) router. Specifically, it removes the unused PDRouterError enum, the api_path utility function, and the BatchRequestWithBootstrap struct from pd_types.rs. In pd_router.rs, it removes the unused _start_time parameter from execute_dual_dispatch_internal (allowing the removal of the clippy argument limit suppression) and cleans up several redundant comments. There are no review comments, so I have no feedback to provide.

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[claude]

👋 The PR description doesn't fully follow
PULL_REQUEST_TEMPLATE.md:

• Missing header: ### Problem (present as inline text "Problem:" but not as a markdown header)
• Missing header: ### Solution (present as inline text "Solution:" but not as a markdown header)

Please update the PR description so reviewers have the context they need.

[claude]

Clean deletion-only PR. Verified all 4 removed items (PDRouterError, api_path, BatchRequestWithBootstrap, _start_time param) are unused repo-wide. Stale comments were correctly identified — get_server_info proxies to prefill, not decode. The clippy expect removal is correct (6 args, under the 7-arg threshold). No issues found.