openapi: unquoted YAML date literals drop defaults and destroy enum values

[OmarAlJarrah]

Summary

YAML resolves plain scalars like 2021-01-01 to the !!timestamp tag. scalarValue has no case for that tag and errors with "unsupported scalar tag", and the failure cascades differently by position:

• default: — dropped with a warning diagnostic.
• example:/examples: — dropped silently (conversion errors are discarded).
• enum: — worst case: one unconvertible member makes enumMembers bail, the enum falls to enumAsUnion, and every date member is re-converted via valueOrNull into a null literal. The actual enum values are destroyed, not preserved: enum: [2021-01-01, 2022-02-02] becomes a union of two literals whose values are both ValueNull.

Dates in defaults/enums are routine spec content, and YAML does not require quoting them.

Reproduction

openapi: 3.1.0
info: {title: t, version: '1'}
paths: {}
components:
  schemas:
    D:
      type: string
      format: date
      default: 2021-01-01
      enum: [2021-01-01, 2022-02-02]

Result: Default=nil; a Union of two Literals both kind=null; diagnostics default: openapi: unsupported scalar tag "!!timestamp" and heterogeneous or non-scalar enum lowered as a union of literals.

Root cause

compilers/openapi/value.go:48-88 (no !!timestamp case in scalarValue); enum destruction via compose.go:379-399 + 437-445 (enumAsUnion + valueOrNull mapping conversion failure to a null literal); silent example drop at schema.go:556-569.

Expected

!!timestamp scalars should be captured as their string spelling (ValueStr) — the schema declares type: string; the tag is a YAML parsing artifact, not source semantics. Independently: a failed value conversion must never silently become a null literal, and example-conversion errors must not be discarded.