AI integration deep proxies return new function references on every property access

The `get` trap in AI integration deep proxies always returns a different value instead of memoizing bound/instrumented methods.

For example, `console.log(ai.run === ai.run)` prints `false` because each access creates a new function via `instrumentMethod(...)` or `value.bind(obj)`.

This pattern exists across multiple AI integrations:

- `packages/core/src/tracing/openai/index.ts` (`createDeepProxy`)
- `packages/core/src/tracing/anthropic-ai/index.ts` (`createDeepProxy`)
- `packages/core/src/tracing/google-genai/index.ts` (`createDeepProxy`)

Suggested fix: keep a `Map` in the closure to store bound/instrumented methods, rather than creating a new value on every `get`.

> Originally posted by @isaacs in https://github.com/getsentry/sentry-javascript/pull/22119#discussion_r3581015061

Start at createDeepProxy in packages/core/src/tracing/openai/index.ts, packages/core/src/tracing/anthropic-ai/index.ts, and packages/core/src/tracing/google-genai/index.ts. Trace the get trap and the instrumentMethod or bind paths, then verify that repeated access to the same method returns the same reference while preserving instrumentation behavior.

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