fix(snapshot): support no-unsafe-eval CSP by evaluating snapshot files server-side

[lazerg]

Description

In browser mode, when the page is served with a no-unsafe-eval CSP policy, getSnapshotData silently fails to parse .snap files because new Function() is blocked. The empty catch {} discards the error, leaving snapshot data empty and producing confusing mismatch failures with no hint about the real cause.

This PR introduces an optional readSnapshotFileData method on SnapshotEnvironment that returns already-evaluated snapshot data. When the environment provides it, SnapshotState.create() uses it instead of readSnapshotFile + new Function, so the browser tester runtime never has to eval. The browser's VitestBrowserSnapshotEnvironment implements the method by delegating to the server via RPC, where the file is read and evaluated safely in Node.js context.

Resolves #10615

[lazerg]

the two windows failures aren't from this PR, both Build&Test and Browsers windows jobs fail the same way on https://github.com/vitest-dev/vitest/actions/runs/27978136425 which is a completely different branch. looks like pre-existing flakiness on windows

[hi-ogawa]

I think we can go all-in as breaking change like:

• require readSnapshotFileData
• surface populate error as hard error and say something like "invalid snapshot file, please manually fix or delete (path to snapshot file)"

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new e2e should cover

• csp wihtout unsave eval
• new hard error populate behavior

[hi-ogawa]

On 2nd thought, it may be fine this is optional for integrator but fill up the default implementation on environment side somehow.