Add explicit schema reference resolution

[ChiragAgg5k]

What does this PR do?

Adds Specification::resolveSchema() for consumers that need the concrete type behind a local component schema reference.

The resolver:

• follows chained OpenAPI 3 #/components/schemas/... references
• supports Swagger 2 #/definitions/... references
• detects cycles
• leaves missing and external references unresolved
• preserves the parser\u2019s existing behavior of retaining ReferenceSchema objects by default

This lets appwrite/sdk-generator infer transport behavior for referenced binary schemas without maintaining its own reference traversal.

Related to appwrite/sdk-generator#1871 and CLO-4377.

Test plan

• composer check

[greptile-apps]

Greptile Summary

Adds an explicit schema-reference resolver to the immutable specification model.

• Resolves chained local OpenAPI 3 component and Swagger 2 definition references.
• Preserves missing, external, and cyclic references.
• Handles percent-encoded local schema names without treating encoded external references as local.
• Documents the new API and adds coverage for resolution behavior.

Confidence Score: 5/5

The PR appears safe to merge; no new actionable issues or outstanding blocking findings remain.

The locality check now occurs before percent-decoding, so encoded external references remain unresolved while encoded local names resolve correctly. The earlier Swagger coverage thread was manually resolved without explanation.

Important Files Changed

Filename	Overview
src/Specification.php	Adds explicit local schema-reference traversal with locality, missing-target, and cycle guards.
tests/ParserTest.php	Covers direct, chained, encoded, missing, external, encoded-external, and cyclic schema references.
README.md	Documents explicit schema resolution while clarifying that unresolved references remain preserved.

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[ChiragAgg5k]

@greptile review