[Config Registry] 6/6 Forbid use of System.Environment methods and adapt everywhere

[anna-git]

Context

Part of Configuration Inversion (Step 6) - Stack progress:

1. #7548 - Add GitLab step and JSON configuration file
2. #7688 - Cleanup configuration / platform keys + source generator
3. #7698 - Aliases handling via source generator
4. #7689 - Analyzers for platform and ConfigurationBuilder
5. #7697 - Replace manual ConfigurationKeys by generated ones in the whole solution (this PR)
6. → #7905 - Forbid use of System.Environment methods and adapt everywhere (this PR)
   I'll update the PR summary to mention the YAML documentation file:

Summary of changes

Banned direct System.Environment.GetEnvironmentVariable() usage and migrated all environment variable access to use EnvironmentHelpers with strongly-typed ConfigurationKeys and PlatformKeys constants.

Key changes:

• Banned System.Environment.GetEnvironmentVariable() via BannedApiAnalyzers
• Added EnvironmentGetEnvironmentVariableAnalyzer (DD0009) to enforce ConfigurationKeys/PlatformKeys usage only
• Nested PlatformKeys by category (Ci, Aws, AzureAppService, ServiceFabric, DotNet)
• Migrated ~50+ files across CI Visibility, AWS Lambda, AppSec, Telemetry, Profiler, and Agent components
• Added missing keys to supported-configurations.json and their documentation to supported-configurations-docs.yaml

Reason for change

Direct System.Environment.GetEnvironmentVariable() calls with string literals are error-prone. Centralizing through EnvironmentHelpers with strongly-typed constants provides compile-time validation, discoverability, and refactoring safety.

Implementation details

1. Banned API enforcement - Added BannedSymbols.txt and configured .editorconfig to treat RS0030 as error (vendored code excluded)
2. Custom analyzer - DD0009 validates all EnvironmentHelpers calls accept only ConfigurationKeys/PlatformKeys constants, rejecting hardcoded strings
3. PlatformKeys organization - Nested by category for better discoverability
4. EnvironmentHelpers refactoring - Added overloads for nested class constants, maintained backward compatibility
5. Configuration documentation - Added missing configuration keys to supported-configurations.json and comprehensive XML documentation to supported-configurations-docs.yaml, which the source generator uses to generate XML doc comments in ConfigurationKeys classes

Test coverage

• ✅ Comprehensive analyzer tests covering valid/invalid scenarios
• ✅ All existing unit tests pass
• ✅ Banned API analyzer correctly flags direct System.Environment usage

Other details

+2,600/-940 lines. No breaking changes, negligible performance impact.

[datadog-official]

⚠️ Tests

⚠️ Warnings

🧪 1 Test failed

❄️ Known flaky: StableConfigWhenProfilingEnabled from Datadog.Profiler.IntegrationTests.SingleStepInstrumentation.SingleStepInstrumentationTest (Datadog)

The test Samples.BuggyBits is running for too long or was lost

ℹ️ Info

❄️ No new flaky tests detected

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