fix(spy): point ESM namespace spy error to module mocking docs

[ryandiginomad]

Description

The Cannot spy on export "…". Module namespace is not configurable in ESM error links users to a browser-mode-only docs anchor (/guide/browser/#limitations). But this error fires in Node too — the reproduction in the issue (vi.spyOn(lodash, 'after')) has nothing to do with browser mode, so the link sends Node users somewhere irrelevant.

This points the message at the environment-agnostic module mocking guide (/guide/mocking/modules#mocking-a-module) instead — the standard vi.mock(path, { spy: true }) solution — following @sheremet-va's steer in the issue to "lean into the standard" rather than suggest a mode-specific config. The message body and the guard are unchanged; only the link moves. No environment branching is added.

I used the #mocking-a-module section anchor (not the bare page) to match the deep-linking convention already used for the sibling module-mocking error in packages/mocker/src/node/hoistMocks.ts.

Resolves #9467

Tests

Verified locally (fix reverted → test fails, fix applied → passes):

• Node (test/unit): added a regression test in vi.spyOn() edge cases that fakes a non-configurable ESM namespace and asserts the message points at the mocking guide and not at /guide/browser. This is the path the browser snapshot test didn't cover.
• Browser (test/browser): updated the existing inline snapshot to the new link.

Open questions for maintainers

• The { spy: true } remedy on the target page currently sits inside a "Browser Mode Support" danger callout — a Node reader landing there might still hesitate. Happy to open a small follow-up to make that section's framing environment-agnostic if you'd like, but kept it out of this PR to keep the change minimal.
• I left the error message body as-is (link only). If you'd prefer it to inline the vi.mock(path, { spy: true }) hint directly, I can do that too — just wanted to avoid over-reaching (the earlier chore: improve spyOn ESM namespace error guidance #9505 went that route).
• The issue also notes the v3→v4 behavior change isn't in the migration guide. That felt separable from this message fix, so I left it out — happy to follow up if useful.

🤖 AI assistance

Developed with Claude Code. I diagnosed the throw site, verified the docs anchor lands on the right content, and confirmed the fix with a local red→green test run. I've reviewed every line of this diff.

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[ryandiginomad]

Quick check-in on this one — it's been in the queue a couple of weeks. I've rebased it on main and CI is green.

It's a small fix following the direction @sheremet-va outlined in #9467: the vi.spyOn ESM namespace error currently points Node users at browser-mode docs, so this points it at the environment-agnostic mocking guide instead. Happy to adjust the wording or the target anchor if you'd prefer something different.