… keys (#7688)

## Context

Part of **Configuration Inversion (Step 2)** - Stack progress:
1. [#7548](#7548) - Add
GitLab step and JSON configuration file
2. **→ [#7688](#7688) -
[Config registry] Reorganize keys, split config keys and platform
keys(this PR)**
3. [#7698](#7698) -
Generate ConfigurationKeys with source generator
4. [#7689](#7689) -
Aliases handling and analyzers
5. [#7697](#7697) -
Replace manual ConfigurationKeys by generated ones in the whole solution
## Summary

Separates platform-specific environment variables into `PlatformKeys`
class and adds Roslyn analyzers to enforce proper usage of configuration
keys throughout the codebase.

## Changes

**Platform Keys Separation:**
- Created `PlatformKeys` class for external platform environment
variables (AWS, Azure, GCP, Kubernetes, CLR Profiler), those that don't
start with DD_ and come from other platforms.
- Updated all references throughout codebase to use `PlatformKeys` for
platform-specific variables
- Keeps `ConfigurationKeys` focused only on Datadog configuration

**Roslyn Analyzers:**
- **DD0007**: Platform key analyzer - Enforces use of `PlatformKeys` for
external platform environment variables
- **DD0008**: ConfigurationBuilder analyzer - Enforces use of
`ConfigurationKeys` or `PlatformKeys` constants in
`ConfigurationBuilder.WithKeys()` calls
- Prevents hardcoded strings and ensures type safety

**Configuration Mapping:**
- Added `configuration_keys_mapping.json` to map environment variable
names to their `ConfigurationKeys` constant names, so that we avoid
renaming hundreds of variables according to automatically source
generated deducted ones.
- Enables preservation of original constant names during refactoring
- Supports source generator integration

**Code Updates:**
- Updated `TracerSettings`, test files, and profiler code to use
`PlatformKeys`
- Replaced manual `ConfigurationKeys` with generated version from
`supported-configurations.json`
- Added missing `DD_TRACE_LOG_PATH` to configuration registry

## Motivation

Ensures clean separation between Datadog and platform configuration by:
- Preventing accidental mixing of platform and Datadog keys
- Enforcing compile-time validation of configuration key usage
- Making it clear which keys are owned by Datadog vs external platforms
- Catching configuration errors at build time instead of runtime

## Validation

- Roslyn analyzers run on every build
- All existing tests pass with updated key references
- Analyzers report clear diagnostic messages for violations

## Related Work

This PR builds on the configuration registry foundation established in
#7548:
- **Commit `2639cf0`** added `supported-configurations.json` with ~280
keys, aliases, deprecations, and GitLab CI validation
- **Commit `70e632e`** added
[supported-configurations-docs.yaml](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Configuration/supported-configurations-docs.yaml:0:0-0:0)
with XML documentation for automated doc generation

…#7698)

## Context

Part of **Configuration Inversion (Step 3)** - Stack progress:
1. [#7548](#7548) - Add
GitLab step and JSON configuration file
2. [#7688](#7688) -
Cleanup configuration / platform keys + analyzers
3. **→ [#7698](#7698) -
Generate ConfigurationKeys with source generator (this PR)**
4. [#7689](#7689) -
Aliases handling via source generator
5. [#7697](#7697) -
Replace manual ConfigurationKeys by generated ones in the whole solution

## Summary

Adds source generator to auto-generate `ConfigurationKeys` from
`supported-configurations.json` with name mapping to preserve existing
constant names.

## Changes

**Source Generator:**
-
[ConfigurationKeysGenerator](cci:7://file:///var/folders/4j/q1f1tvc503g_thnq6p79cp500000gp/T/SourceGeneratedDocuments/D0E9C9C62821E2CB6FADE080/Datadog.Trace.SourceGenerators/ConfigurationKeysGenerator:0:0-0:0)
reads `supported-configurations.json` and
[supported-configurations-docs.yaml](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Configuration/supported-configurations-docs.yaml:0:0-0:0)
- Auto-generates `ConfigurationKeys` with nested product classes
(AppSec, CIVisibility, OpenTelemetry, etc.)
- Generates XML documentation and `[Obsolete]` attributes from JSON

**Configuration Mapping:**
- `configuration_keys_mapping.json` maps env vars to original constant
names (e.g., `DD_TRACE_ENABLED` → `TraceEnabled`)
- Avoids refactoring hundreds of references across codebase

**Updates:**
- Added missing `DD_TRACE_ACTIVITY_LISTENER_ENABLED` key
- Committed temporary
[ConfigurationKeys2.g.cs](cci:7://file:///Users/anna.yafi/go/src/github.com/DataDog/dd-trace-dotnet3/tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/ConfigurationKeysGenerator/ConfigurationKeys2.g.cs:0:0-0:0)
files for validation

## Motivation

Makes `supported-configurations.json` the single source of truth while
preserving existing constant names to avoid massive refactoring.

## Test Coverage

- Added `ConfigurationKeysGeneratorTests.cs` with comprehensive unit
tests
- Tests validate generator output, XML documentation generation, and
obsolete attribute application
- Tests cover nested product classes, deprecations, and YAML
documentation integration
- All existing tests pass without modification

## Related Work

Builds on #7548 (configuration registry) and #7688 (PlatformKeys
separation + analyzers).