feat(vtz): add vitest-compat mock APIs to @vertz/test (toward #2731)

[viniciusdacal]

Summary

Real-world test suites written against vitest often call getMockImplementation(), getMockName(), mockName(), and withImplementation() on mock functions. vtz's runtime exposed mock() / vi.fn() without those methods, so tests migrated from vitest hit TypeError: x.getMockImplementation is not a function (surfaced in #2731).

This PR fills that part of the gap. It is not about transitive-mock propagation — another agent is handling that.

New APIs on every mock

• getMockImplementation() — returns the current default implementation, or undefined if none is set. Does not consider the once-queue (matches vitest).
• getMockName() — returns the display name set via mockName(). Defaults to ''.
• mockName(name) — sets the display name for diagnostics. Returns the mock for chaining. Cleared by mockReset(); preserved by mockClear().
• withImplementation(fn, cb) — temporarily swaps the default implementation with fn, runs cb, then restores the original — awaiting cb if it returns a Promise. Returns cb's result. Restores on both sync and async exceptions. Leaves getMockImplementation() unchanged after return.

Exposed on mock(), vi.fn(), and spyOn().

Not implemented (intentional)

• mockThrow / mockThrowOnce (v4.1.0+ vitest) — adds surface without strong use-case yet. Easy to add later.
• mock.settledResults, mock.instances, mock.contexts, mock.invocationCallOrder — separate state streams the runtime doesn't currently track. Follow-up if demand materializes. Documented in the changeset.

Public API changes

Additive to MockFunction interface in @vertz/test/src/index.ts:

mockReturnThis(): MockFunction<TArgs, TReturn>;
getMockImplementation(): ((...args: TArgs) => TReturn) | undefined;
getMockName(): string;
mockName(name: string): MockFunction<TArgs, TReturn>;
withImplementation<R>(fn: (...args: TArgs) => TReturn, cb: () => R): R;

No breaking changes. No deferred changes.

Test plan

• 10 Rust unit tests in native/vtz/src/test/globals.rs covering all four new methods + mockReset/mockClear interactions + sync/async cb restoration + exception paths.
• 15 TypeScript tests in packages/test/src/__tests__/mock-api.test.ts exercising the full surface via the public @vertz/test import.
• cargo test -p vtz --lib — 3453 passed.
• cargo clippy -p vtz --all-targets -- -D warnings — clean.
• cargo fmt --all -- --check — clean.
• vtz test packages/test/src/__tests__/mock-api.test.ts — 15/15 pass.
• vtz run typecheck on @vertz/test — clean.

Relates to #2731.

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