[codex] Add writable INI configuration provider

[sharpninja]

Summary

Adds writable INI support to Microsoft.Extensions.Configuration.Ini while preserving the existing read-only provider behavior.

The change introduces:

• IniDocument, a lossless editable INI document model for section/key/value order and quote preservation
• WritableIniConfigurationSource and WritableIniConfigurationProvider
• AddWritableIniFile(...) extension API
• tests for round-trip preservation, read parity, optional missing files, and provider save behavior
• reference assembly entries for the new public API surface

Motivation

The built-in INI provider can read files but cannot write changes back. This adds a writable provider for scenarios where applications need to read, modify, and save shared INI configuration without dropping unmanaged keys or changing quote style for existing values.

Validation

• git diff --check
• eng\common\dotnet.cmd build src\libraries\Microsoft.Extensions.Configuration.Ini\tests\Microsoft.Extensions.Configuration.Ini.Tests.csproj -c Debug -f net11.0 -v minimal /p:UseLocalTargetingRuntimePack=false

The test assembly builds successfully. Direct local test execution is not included here because the runtime testhost was not built in this checkout; build.cmd libs.pretest -c Debug -f net11.0 /p:UseLocalTargetingRuntimePack=false stopped on the standard prerequisite that the CoreCLR subset must be built first.

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[rosebyte]

@sharpninja, please create an issue with a proposal first as we need to get API approval before anything can be merged.

[sharpninja]

@dotnet-policy-service agree

[sharpninja]

Created the requested API proposal issue: #128989