fix(solid-query): prevent lost updates when mounting cached queries

[2wheeh]

fixes #11384

🎯 Changes

Fixes a Solid Query timing issue where updates could be lost when a cached query refetched during mount.

Observer attachment is deferred until the conditional subtree is ready, while the optimistic fetch status remains available synchronously. Regression tests cover the behavior in both browser and semantic tests.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested code changes locally with pnpm run test:pr, or these tests do not apply to this pull request.
• I fully understand the code in this pull request, including any code generated with AI assistance.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.

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