openapi: a percent-encoded $dynamicRef fragment does not match its $dynamicAnchor

[OmarAlJarrah]

A $dynamicRef fragment is URI text, so #my%2Danchor names the anchor my-anchor. The
anchor index compares the raw fragment against declared $dynamicAnchor names instead, so a
spec-correct escape matches nothing: the reference is reported irreducible and kept verbatim
under Unmodeled rather than expanded to the type it names.

Reproduction

openapi: 3.1.0
info: {title: t, version: "1"}
paths: {}
components:
  schemas:
    A:
      type: object
      properties:
        x: {$dynamicRef: "#my%2Danchor"}
    B: {$dynamicAnchor: my-anchor, type: string}

Emitted:

openapi/degraded-construct: $dynamicRef was not expanded because no $dynamicAnchor
"my%2Danchor" is declared in this document; it is kept verbatim under Unmodeled

Spelling the same reference #my-anchor expands it to t/openapi/components/schemas/B.

Root cause

dynamicFragment (compilers/openapi/internal/schema/schema.go) cuts the leading # and
returns the remainder as the anchor name, never percent-decoding it. soleAnchorSite then looks
that raw text up in the $dynamicAnchor index, which is keyed by the names as declared.

Why this is separate from #40

#40 covers $ref pointers, where the fix is to read the fragment the way the library that
actually resolves the reference reads it — disagreeing with the resolver is what made it a bug.
Nothing external resolves $dynamicRef: this compiler owns the anchor index and the match, so
what the decoding should be is a decision about our own behavior rather than one taken by
matching a dependency. Worth settling on its own terms, including whether $anchor names should
be decoded on the declaration side too.