fix: undo strict null widening for tool inputs

[jan-kubica]

🎯 Changes

OpenAI-compatible strict function schemas promote optional properties to required nullable properties. The model can therefore return null for an omitted optional field, but TanStack AI subsequently validates that value against the original Standard Schema, where .optional() accepts absence/undefined, not null. This prevents otherwise valid server tools from executing.

This PR:

• records a NullWideningMap during the same OpenAI strict-schema conversion that adds nullability
• removes only provider-synthesized nulls before emitting completed tool input, while preserving nulls the original schema genuinely allows
• applies the normalization to both Chat Completions and Responses, including their completion/backfill paths
• leaves non-strict tools and ambiguous duplicate tool names untouched
• adds adapter unit coverage plus a mocked Responses wire round trip that validates and executes the original Zod tool schema across two model turns

Test plan

• pnpm run test:pr
• pnpm --filter @tanstack/openai-base test:lib (134 tests)
• pnpm --filter @tanstack/ai-e2e test:e2e -- openai-strict-tool-null-wire.spec.ts
• pnpm --filter @tanstack/ai-e2e test:e2e (347 passed, 2 unrelated flaky tests passed on retry, 1 gated live smoke skipped)

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

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