Fix numeric enum deserialization

[sakshichitnis27]

Fixes #24344

Description

Makes numeric inner-enum deserialization consistent with string enums in the C# generichost generator.

• Unknown nullable numeric enum values are deserialized as null.
• Unknown non-nullable numeric enum values leave the option unset.
• Applies the same behavior to byte-backed numeric enums.
• Adds regression coverage for nullable and non-nullable numeric enums.

Validation

• CSharpClientCodegenTest: 10 tests passed, 0 failures.
• Generated a fresh .NET 8 client from the test fixture.
• Verified at runtime that an unknown nullable value becomes null.
• Verified at runtime that an unknown non-nullable value remains unset.
• git diff --check passed.

PR checklist

• Read the contribution guidelines.
• Ran the complete project build and regenerated all samples. Focused generator tests and runtime validation were run instead.
• C# technical committee: @mandrean @shibayan @Blackclaws @lucamazzanti @iBicha

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Summary by cubic

Fix numeric enum deserialization in the C# generichost generator to match string enums. Unknown numeric values now become null for nullable enums and are skipped for non‑nullable enums (including byte‑backed); parsing is centralized in NumericInnerEnumJsonRead.mustache, with tests covering nullable inline int enums.

^{Written for commit ab54d9a. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 3 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[wing328]

thanks for the PR

cc @devhl-labs

[wing328]

please follow step 3 to update the samples

[cubic-dev-ai]

All reported issues were addressed across 2 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>