fix(pd): route chat using full message history

[Moersity]

Description

Closes #1928

Problem

The HTTP PD router only inspected the first chat message when building routing text. For multi-turn conversations, later messages did not participate in cache-aware or prefix-based worker selection. Multipart text in the first message could also produce no routing text.

Solution

Use ChatCompletionRequest::extract_text_for_routing(), matching the regular HTTP router and including text from the complete message history.

Changes

• Replace first-message-only extraction in the HTTP PD chat route.
• Remove the now-unused ChatMessage and MessageContent imports.

Test Plan

1. Send two multi-turn chat requests with the same first message but different later messages through HTTP PD routing.
2. Confirm the routing text differs and includes the complete message history.
3. Send a request containing multipart text and confirm its text participates in routing.
4. Run cargo test -p smg --lib routers::http::pd_router::tests (8 passed, 0 failed).
5. Run cargo +nightly fmt --all -- --check (passed).

cargo clippy --workspace --all-targets -- -D warnings is currently blocked on macOS by two pre-existing unused imports in model_gateway/src/routers/grpc/multimodal/assemble.rs. The all-features variant additionally requires local OpenCV/pkg-config.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes (blocked by the environment/pre-existing warnings described above)
• (Optional) Documentation updated (not required; no user-facing API change)
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

Summary by CodeRabbit

• Refactor
  • Streamlined chat request text extraction for HTTP routing.
  • No user-visible behavior changes.

[coderabbitai]

📝 Walkthrough

Walkthrough

The HTTP chat router now obtains routing text through body.extract_text_for_routing() instead of manually scanning messages, with corresponding protocol import updates.

Changes

Chat routing

Layer / File(s)	Summary
Use shared routing text extraction model_gateway/src/routers/http/pd_router.rs	Updates protocol imports and replaces manual message text extraction in route_chat with body.extract_text_for_routing().

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: key4ng, slin1237

Poem

I’m a rabbit routing text with care,
No message-scanning maze to wear.
One shared extractor, swift and bright,
Sends chat requests along just right.
Hop, hop—imports are tidy tonight!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: routing chat using the full message history.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[gemini-code-assist]

Code Review

This pull request simplifies the routing text extraction logic in pd_router.rs by replacing manual pattern matching on ChatMessage with a call to body.extract_text_for_routing(). Feedback was provided to prevent potential load imbalance issues by ensuring that empty strings returned by extract_text_for_routing() are mapped to None instead of being wrapped in Some("").

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[gemini-code-assist]

If extract_text_for_routing() returns an empty string, wrapping it in Some results in Some(""). Passing an empty string to routing policies (like cache-aware or consistent hashing) can cause all requests with empty routing text to hash to the same worker, leading to load imbalance. It is better to map an empty string to None so that policies can correctly fall back to load-balancing or round-robin routing.

Suggested change

	let request_text = if self.policies_need_request_text() {
	body.messages.first().and_then(|msg| match msg {
	ChatMessage::User { content, .. } => match content {
	MessageContent::Text(text) => Some(text.clone()),
	MessageContent::Parts(_) => None,
	},
	ChatMessage::Developer { content, .. } => match content {
	MessageContent::Text(text) => Some(text.clone()),
	MessageContent::Parts(_) => None,
	},
	ChatMessage::System { content, .. } => Some(content.to_simple_string()),
	_ => None,
	})
	Some(body.extract_text_for_routing())
	} else {
	None
	};
	let request_text = if self.policies_need_request_text() {
	let text = body.extract_text_for_routing();
	(!text.is_empty()).then_some(text)
	} else {
	None
	};