Add $ref support to all four language code generators (#1062)

* Add $ref support to all four language code generators

Enable JSON Schema $ref for type deduplication across all SDK code
generators (TypeScript, Python, Go, C#).

Changes:
- utils.ts: Add resolveRef(), refTypeName(), collectDefinitions() helpers;
  normalize $defs to definitions in postProcessSchema
- typescript.ts: Build combined schema with shared definitions and compile
  once via unreachableDefinitions, instead of per-method compilation
- python.ts/go.ts: Include all definitions alongside SessionEvent for
  quicktype  resolution; include shared API defs in RPC combined schema
- csharp.ts: Add  handling to resolveSessionPropertyType and
  resolveRpcType; generate classes for referenced types on demand

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Align rebased ref generator changes

Keep the rebased $ref generator follow-up aligned with the latest C# typing changes and clean up the Python/TypeScript generator adjustments.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Handle mixed schema definitions in codegen

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Refresh $ref codegen for latest schema

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix ref codegen review issues

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub