[sgl-router] refactor - SLO ordering for bucket selection

[sherlockwu]

Add optional TTFT and token-throughput preferences to bucket selection. Length constraints remain mandatory; SLO preferences order the compatible buckets before engine selection. Each PD attempt still obtains both engines from one bucket, and nonpreferred buckets remain available for fallback.

Based directly on main; the cache-aware changes from #40366 and admission changes from #40271 are already merged.

Changes

• Add optional service estimates to Bucket and independent resolver preferences: Disabled (default), SloFirst (matching first), or BestEffort (nonmatching first).
• Weight unmet TTFT and throughput preferences equally, then break ties by capacity, rank, and ID. With both set to SloFirst, buckets matching both targets precede those matching one, followed by those matching neither.
• Treat absent targets as neutral and missing or invalid estimates as nonmatching. Preserve input/context limits and admission fallback across SLO tiers.
• Pass targets directly to BucketResolver::resolve. Parse x-sgl-ttft-slo-ms and x-sgl-tps-slo in the shared chat route only when their preference is enabled; invalid enabled headers return 400 before dispatch, while disabled headers are ignored.
• Cover conflicting targets, mixed preferences, missing estimates, invalid inputs, length constraints, complete P/D pairing, and rejection fallback.

Preferences are configured programmatically on reorg resolvers. Legacy routing remains the default; production configuration construction remains a follow-up.

Validation after rebase

Rebased only the SLO commit onto main (9eda772a21), excluding the already-merged cache-aware and admission commits. Preserve BucketResolver::new stage validation and its Result return type from #40366; adapt SLO test constructors to that API. Cache-aware and admission policy implementations and their regression tests remain identical to main.

• cargo fmt --all --check
• cargo test --locked --workspace --lib --tests --quiet: 1,117 tests passed, none skipped.
• cargo clippy --locked --workspace --all-targets -- -D warnings
• git diff --check

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CI States

Latest PR Test (Base): ✅ Run #35678290678
Latest PR Test (Extra): ❌ Run #35678290559
Latest PR Test (AMD ROCm 10): ➖ No AMD PR run found for this commit.

[ShangmingCai]

Logic is correct and genuinely well-tested, after a restack/rebase, we could launch the CI and merge it after CI passes.

[ShangmingCai]

Be careful with this part when restacking, since #40366 and #40271 are already in main