refactor(api): use canonical provider identifiers

[WebMad]

Summary

• replace raw provider string literals in buildApiHandler() with canonical providerIdentifiers
• use retiredProviderIdentifiers.roo for the dedicated Roo removal path
• preserve existing handler routing, retired-provider errors, and unsupported-provider fallback behavior
• expand focused handler-factory coverage for representative active, retired, and unsupported providers

Closes #955

Test plan

• cd src && npx vitest run api/__tests__/index.spec.ts api/providers/__tests__/friendli.spec.ts — 28 passed
• cd src && npx vitest run api — 2,035 passed, 2 skipped
• cd src && pnpm check-types
• repository pre-commit lint — 11 packages passed
• pre-push repository type checks — 11 packages passed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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