feat(compiler): add csharp target and idl integration tests

[chaokunyang]

Why?

Add full C# schema IDL support end-to-end in Apache Fory: compiler target, documentation, runtime compatibility fixes, and integration coverage (including root/package cross-reference and file-based roundtrip scenarios).

What does this PR do?

• Adds C# as a first-class compiler target:
  • foryc --lang ... ,csharp
  • foryc --csharp_out=<dir>
  • option csharp_namespace = "..."
• Implements C# schema generator (compiler/fory_compiler/generators/csharp.py) for messages/enums/unions, registration helpers, and generated ToBytes/FromBytes helpers.
• Wires C# into compiler generator registration and generated-code test matrix.
• Adds C# generator unit tests (compiler/fory_compiler/tests/test_csharp_generator.py) covering namespace resolution, registration behavior, field encoding attributes, imported registration calls, and parser option support.
• Adds full C# IDL integration test harness under integration_tests/idl_tests/csharp/IdlTests and runner integration_tests/idl_tests/run_csharp_tests.sh, including schema-consistent and compatible roundtrips across:
  • addressbook, auto_id, complex_pb primitives
  • collection union/array variants
  • optional_types
  • any_example (.fdl) and any_example (.proto)
  • flatbuffers (monster, complex_fbs)
  • reference-tracking models (tree, graph)
  • evolving compatibility cases
  • root/package cross-reference (root.idl) and generated bytes helper paths
• Adds C# IDL output target to integration_tests/idl_tests/generate_idl.py.
• Improves C# runtime compatibility behavior for generated IDL shapes:
  • collection/dictionary serializers now handle declared-type metadata correctly in compatible mode.
  • union serializer now supports typed case handling while preserving xlang-compatible dynamic framing and case value normalization.
• Updates compiler documentation (compiler/README.md, docs/compiler/*) and C# README to document C# codegen, options, generated output, and test workflow.

Related issues

#1017
#3387

Does this PR introduce any user-facing change?

Yes. foryc now supports C# generation (--lang csharp, --csharp_out) and C# namespace override (csharp_namespace), with updated compiler docs and C# IDL test workflow.

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

N/A (feature + compatibility work; no benchmark updates in this PR)