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Generated TryGetIndex counts only index digits, but collection propagators slice past the entire [index] segment. This forwards malformed child paths and breaks nested JsonPatch reads and writes.

• Generator: Include both brackets in successful bytesConsumed; preserve parse-failure behavior.
• Snapshots: Update the helper baseline and generated sample.
• Regression coverage: Exercise single- and multi-digit indices, invalid indices, nested lists, and parent-to-child patch reads and writes.

Input:             [12].description
bytesConsumed:     2 → 4
Forwarded path:    $2].description → $.description

• Fixes [C#] Incorrect bytesConsumed in generated TryGetIndex breaks nested JsonPatch propagation through collections #12431

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[Copilot]

🟢 Approval recommended

No active review issues were identified for the collection-path forwarding fix.

No open issues 🎉

What changed in this PR

This PR fixes collection-path forwarding in generated C# dynamic models, addressing #12431 so nested JsonPatch reads and writes receive the correct child path.

Changes:

• Count both brackets in successful TryGetIndex consumption while preserving parse-failure behavior.
• Update the generated sample and helper baseline.
• Add regression tests for index parsing and parent-to-child patch propagation through single and nested collections.

File	Description
packages/​http-client-csharp/​generator/​TestProjects/​Local/​Sample-TypeSpec/​src/​Generated/​Internal/​ModelSerializationExtensions.cs	Updates the generated helper’s consumed-byte count.
packages/​http-client-csharp/​generator/​TestProjects/​Local.Tests/​ModelSerializationExtensionsTests.cs	Tests valid indices, remaining paths, and invalid inputs.
packages/​http-client-csharp/​generator/​Microsoft.TypeSpec.Generator.ClientModel/​test/​Providers/​Definitions/​TestData/​ModelSerializationExtensionsDefinitionTests/​ValidateTryGetIndex.cs	Updates the expected generated helper baseline.
packages/​http-client-csharp/​generator/​Microsoft.TypeSpec.Generator.ClientModel/​test/​ModelReaderWriterValidation/​TestProjects/​Sample_TypeSpec/​DynamicModelTests.cs	Tests collection-based patch reads and writes with single- and multi-digit indices.
packages/​http-client-csharp/​generator/​Microsoft.TypeSpec.Generator.ClientModel/​src/​Providers/​ModelSerializationExtensionsDefinition.cs	Emits the bracket-inclusive consumed-byte count after successful parsing.

_{Review effort: Balanced}

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commit: 65f3c2d

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🟢 Approval recommended

No active review issues remain for the collection-index consumption fix supporting nested JsonPatch operations.

No open issues 🎉

_{Review effort: Balanced}