feat: expose open string enum branches

[ChiragAgg5k]

What

Expose the enum-bearing branch when an anyOf combines exactly one string enum with one or more enum-free string branches.

This shape documents the values callers commonly use without closing the accepted string set:

{
  "anyOf": [
    {
      "type": "string",
      "enum": ["network.requests", "network.inbound"]
    },
    { "type": "string" }
  ]
}

Consumers can now use the canonical model directly:

$enum = $schema->openStringEnumBranch();

instead of duplicating composition and branch inspection.

Behavior

The query is deliberately narrow and returns null unless:

• the composition is anyOf;
• every member is a StringSchema;
• exactly one member carries enum values; and
• at least one other string member has no enum values.

Branch order does not matter. oneOf, allOf, mixed-type unions, closed enums, and unions containing multiple enums remain unchanged.

Context

This semantic query is needed by appwrite/sdk-generator#1816. Keeping it in the canonical OpenAPI model prevents individual generators from maintaining their own schema-shape parsers and lets them focus on language-specific rendering.

Test plan

• branch order is ignored;
• oneOf and allOf are rejected;
• missing enum or open branches are rejected;
• multiple enum branches are rejected;
• integer enum and mixed string/integer unions are rejected;
• full composer check passes: 48 tests, 382 assertions.

[greptile-apps]

Greptile Summary

The PR adds a narrowly scoped semantic query for retrieving the enum-bearing branch from an open string anyOf.

• Adds CompositeSchema::openStringEnumBranch() with composition, type, and branch-count checks.
• Documents the public API and its exclusions.
• Adds tests for branch ordering, unsupported compositions, missing branches, multiple enums, and mixed schema types.

Confidence Score: 5/5

The PR appears safe to merge with no actionable defects identified.

The helper enforces the documented anyOf, direct string-member, unique enum, and open-branch constraints, and the tests exercise the important accepted and rejected shapes.

Important Files Changed

Filename	Overview
src/Model/CompositeSchema.php	Adds the open-string-enum branch query with behavior consistent with the stated direct-StringSchema contract.
tests/Schema/ReaderTest.php	Covers the principal successful shape and the documented rejection cases.
README.md	Documents usage and the major boundaries of the new semantic query.

_{Reviews (1): Last reviewed commit: "feat: expose open string enum branches" | Re-trigger Greptile}