fix(cli): deduplicate router candidates

[alexanderludwig]

Summary

• Router candidate deduplication: A seller advertising one model under multiple aliases is now sent to the routing service once, keyed by the resolved seller/provider/service destination.
• Cheapest offer selection: The buyer keeps the lowest combined input/output price for duplicate aliases.
• Regression coverage: Added tests covering duplicate removal, cheapest-price selection, seller separation, and router option propagation.
• Release timing: This remains a code-only fix and will ship with the next VPR/CLI version bump.

Test Coverage

• Focused router execution suite: 16 passed, 0 failed.
• CLI typecheck: passed.
• CLI build: passed.
• The broad CLI suite was also attempted; unrelated gateway/auth/channel environment-dependent tests fail in the current local setup.

Pre-Landing Review

No issues found. Independent Codex review found no actionable findings.

Design Review

No frontend files changed — design review skipped.

Eval Results

No prompt-related files changed — evals skipped.

Plan Completion

No plan file detected.

Verification Results

3 PASS (CLI typecheck, CLI build, focused router tests), 0 FAIL, 0 SKIPPED.

Documentation

Updated the Unreleased changelog entry to describe the router candidate deduplication behavior.

Test plan

• CLI typecheck passes
• CLI build passes
• Focused router execution tests pass (16/16)
• Broad CLI suite: attempted; unrelated environment-dependent failures remain

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