fix: normalize claude version separators across providers in fuzzyMatchModel

[mrm007]

Summary

• Generalize normalizeModelName in model-availability.ts to handle all claude version numbers, not just 4.5
• Collapse two regex replacements into one using [.-] character class
• Add tests for cross-provider model matching and non-claude safety

Problem

models.dev uses different version separators per provider for the same model:

• anthropic → claude-opus-4-6 (hyphens)
• github-copilot → claude-opus-4.6 (dots)

The fallback chains include github-copilot as a provider alongside anthropic, but fuzzyMatchModel only normalized 4-5↔4.5. All other versions (4-6, 4-0, 4-1) silently failed to match, causing fallback chains to skip copilot entirely and land on lower-quality free models.

Changes

src/shared/model-availability.ts — Replace two hardcoded 4-5 regexes with one generic pattern:

// Before
.replace(/claude-(opus|sonnet|haiku)-4-5/g, "claude-$1-4.5")
.replace(/claude-(opus|sonnet|haiku)-4\.5/g, "claude-$1-4.5")

// After
.replace(/claude-(opus|sonnet|haiku)-(\d+)[.-](\d+)/g, "claude-$1-$2.$3")

src/shared/model-availability.test.ts — 2 new tests:

1. Cross-provider matching: claude-opus-4-6 matches github-copilot/claude-opus-4.6
2. Safety: non-claude hyphens (grok-code-fast-1) are not affected

Testing

All 50 tests pass locally (bun test src/shared/model-availability.test.ts).

Closes #1679

[github-actions]

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[mrm007]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[code-yeongyu]

Closing: superseded by #1855 which provides a more comprehensive refactor. Source branch also deleted. Thanks!