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
Custom headers configured for remote MCP servers were being silently
dropped when OAuth was enabled (the default). This test ensures headers
are always sent to MCP servers regardless of OAuth configuration.
