[router] Add builder pattern for RouterConfig with zero duplication

[slin1237]

Summary

Implements a fluent builder pattern for RouterConfig that eliminates code duplication and simplifies config construction across the codebase.

Key Changes

1. Zero-Duplication Builder

Builder now wraps RouterConfig directly instead of duplicating all 70+ fields:

#[derive(Debug, Clone, Default)]
pub struct RouterConfigBuilder {
    config: RouterConfig,  // Single source of truth
}

2. Setters

• Added .maybe_*() methods for optional fields (eliminates if-let chains)
• Added conditional boolean setters (eliminates if statements)

3. Replaced 32 Manual Constructions

Replaced verbose manual config construction with builder pattern across:

• All config tests (9 in types.rs)
• lib.rs and main.rs
• All test files (20 constructions)

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.

[gemini-code-assist]

Summary of Changes

Hello @slin1237, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the configuration management for the router by implementing a fluent builder pattern for the RouterConfig struct. This change aims to improve code clarity, reduce boilerplate, and enhance maintainability by centralizing configuration logic and simplifying how RouterConfig instances are created and modified throughout the application.

Highlights

• New Builder Pattern for RouterConfig: Introduced a fluent builder pattern for RouterConfig to streamline its construction and configuration.
• Zero-Duplication Implementation: The RouterConfigBuilder now directly wraps the RouterConfig struct, eliminating the need to duplicate its 70+ fields and ensuring automatic synchronization.
• Enhanced Setter Methods: Added convenient .maybe_*() methods for optional fields to avoid verbose if-let chains, and conditional boolean setters to simplify flag management.
• Widespread Adoption: Replaced 32 instances of manual RouterConfig construction across the codebase, including config tests, lib.rs, main.rs, and various test files, with the new builder pattern.

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[gemini-code-assist]

Code Review

This pull request introduces a well-designed builder pattern for RouterConfig, significantly improving configuration ergonomics and maintainability across the codebase. By wrapping the RouterConfig struct, the builder avoids field duplication and provides a fluent, expressive API. The inclusion of maybe_* setters for optional fields and conditional boolean setters is a thoughtful touch that simplifies config construction. The refactoring has been applied consistently, cleaning up numerous manual struct instantiations in tests and application logic. I have one suggestion to further simplify the builder's usage in a test case. Overall, this is an excellent contribution to the project's code quality.

[gemini-code-assist]

The match statement here is unnecessarily complex. Since you already have the mode and policy variables from the test_cases loop, you can directly use the .mode() and .policy() setters on the builder. This avoids deconstructing mode only to reconstruct it again, making the code simpler and more direct.

            let config = RouterConfig::builder()
                .mode(mode)
                .policy(policy)
                .host("127.0.0.1")
                .port(3001)
                .max_payload_size(1024 * 1024)
                .request_timeout_secs(60)
                .worker_startup_timeout_secs(10)
                .worker_startup_check_interval_secs(1)
                .max_concurrent_requests(64)
                .queue_timeout_secs(60)
                .build_unchecked();