fix(provider): coerce numeric tool call IDs for OpenAI-compatible providers

[Qiiks]

Issue for this PR

Fixes #19947

Type of change

• Bug fix

What does this PR do?

Some OpenAI-compatible providers (e.g. NVIDIA NIM kimi-k2.5) return numeric tool call IDs instead of strings, violating the OpenAI API spec. This causes Zod validation errors when the AI SDK processes responses.

The fix adds coercion at two points:

1. Fetch interceptor (provider.ts): For @ai-sdk/openai-compatible responses, numeric IDs in tool_calls[].id and delta.tool_calls[].id are converted to strings before the AI SDK processes them. Both JSON and SSE streaming responses are handled.

2. Part hydration (message-v2.ts): For backwards compatibility, numeric callID values stored in existing sessions are coerced to strings when loaded from the database.

The coercion functions were extracted to a shared utility (src/util/coerce-tool-call-ids.ts) to keep them usable across layers.

How did you verify your code works?

• 16 unit tests covering coerceNumericToolCallIds and transformSSEStream (numeric coercion, string passthrough, nested objects, delta.tool_calls, SSE parsing, [DONE] passthrough, invalid JSON passthrough, multiple events)
• bun typecheck passes with zero errors
• Existing provider tests still pass (258/260, 2 pre-existing timeouts unrelated to this change)
• Verified the coercion mutates the object correctly through type assertions

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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