Ingestion: SpecReader drops reader warnings and opens the file outside the ingestion boundary #181

What

Two defects in tools/OpenCode.Sdk.Tools/Generator/Ingestion/SpecReader.cs:

1. Warnings are dropped. After the OpenAPI reader returns, only diagnostic.Errors become
   ingestion errors (:48-51). A document the reader warns about proceeds without a trace.
2. The open escapes the boundary. File.OpenRead (:24) runs before the try block that
   translates reader failures (:31-44). A spec file that exists but cannot be opened (locked, no
   permission) throws a raw filesystem exception instead of a structured ingestion error.

Why in 1.0

These are defects today, not latent walls. The generator's contract is that every input problem
ends as a located ingestion error, and the 1.0 surface is generated through this reader. Both fixes
are small.

Evidence

• SpecReader.cs:24, :31-44, :48-51.
• Close ingestion walls for typed schema and OpenAPI host semantics #53, rows "warning-only reader diagnostic" and "existing spec file that fails OpenRead"; its
  2026-09-27 status confirmed both on master.

Done when

• Red tests first: a document with a reader warning, and a file that exists but cannot be opened.
  Both end in the structured ingestion failure, and cancellation still surfaces as cancellation.
• The two Close ingestion walls for typed schema and OpenAPI host semantics #53 rows are ticked with a link to this issue.