fix(mcp): tolerate invalid tool output schemas

Closes #26529.

When an MCP server returns a tool whose \`outputSchema\` contains a
broken \`$ref\` (e.g. Google Stitch's \`#/$defs/ScreenInstance\`), the
SDK's typed \`listTools()\` validator throws and opencode marks the
ENTIRE server as failed — losing every other valid tool the server
exposes.

Catch the schema-reference errors and retry with a tolerant schema
(\`looseObject\` + \`outputSchema: z.unknown().optional()\`) via the raw
\`request\` path so the bad tool's schema is accepted as opaque while
the others load normally.

Equivalent fix shape to #26530 (nicolascancino) — kept his approach
since it's correct. Bundles our reproducer test from
\`kit/issue-reproducers\` so the regression is locked in.

Verified red → green → red → green:
- pre-fix: server marked \`failed\`
- post-fix: server stays \`connected\`, valid tool present