Deprecate ReducerContext::identity in favor of database_identity

[Mr-Dust0]

Summary

• add ReducerContext::database_identity() in Rust bindings
• deprecate ReducerContext::identity() and keep it as a compatibility alias
• update reducer docs example to use ctx.database_identity()
• add C# reducer-context equivalent: DatabaseIdentity and obsolete Identity alias
• update Rust/C# module test callsites to the new API name
• update C# codegen snapshots for generated ReducerContext API output

Why

Issue #3201 reports user confusion between reducer ctx.identity() (database/module identity) and ctx.sender. This change clarifies naming while preserving compatibility.

Validation

• cargo check -p spacetimedb -p module-test (Passed)
• dotnet test crates/bindings-csharp/Codegen.Tests/Codegen.Tests.csproj --nologo (Passed)
• dotnet test crates/bindings-csharp/Runtime.Tests/Runtime.Tests.csproj --nologo (Failed) pre-existing unrelated failure:
  • Runtime.Tests/JwtClaimsTest.cs(10,23): CS1729: 'JwtClaims' does not contain a constructor that takes 2 arguments

Compatibility

• Rust: identity() still works but is deprecated in favor of database_identity().
• C#: Identity still works but is marked [Obsolete] in favor of DatabaseIdentity.

Closes #3201

[CLAassistant]

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