docs: document v1 ZodObject-vs-raw-shape failure modes

[Parithosh-Varma]

Implements both docs suggestions from #2627 (reporter offered either; doing both, 24 lines).

Changes

• docs/troubleshooting.md: new entry keyed on the searchable crash (MCP error -32603: Cannot read properties of null (reading '_def')), covering the silent empty-schema variant, the version matrix, and the unwrap-to-raw-shape fix with diff.
• docs/migration/upgrade-to-v2.md: one-paragraph warning under Standard Schema objects that applying the v2 style backwards on v1 crashes (<=1.21) or silently empties the schema (<=1.26).

Validation

• prettier --check clean on both files; pnpm sync:snippets --check passes (no snippet blocks touched).

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commit: 3c4a392