fix(providers): parse self-hosted context metadata

[Aaronontheweb]

Original Problem

OpenAI-compatible self-hosted providers could successfully list models, but Netclaw did not surface the model context window during discovery. In the CLI this showed up as a missing context window, and the runtime could fall back to the default 32K context value even when the backend reported a much larger effective context.

The issue was caused by metadata shape differences between self-hosted backends:

• vLLM reports context length as max_model_len on /v1/models entries.
• llama.cpp reports runtime context as meta.n_ctx on /v1/models entries.
• Recent llama.cpp /props responses can report runtime context as default_generation_settings.n_ctx instead of the older default_generation_settings.params.n_ctx shape.

Netclaw already had separate backend strategies for vLLM and llama.cpp, but the llama.cpp strategy only understood the older n_ctx_train and nested /props shape. n_ctx_train is the training context, not necessarily the runtime-effective context when the server splits capacity across slots.

What Changed

• OpenAI-compatible model discovery now reads context metadata for self-hosted backends without adding those backend-specific fields to the generic OpenAI-style parser.
• vLLM discovery reads max_model_len.
• llama.cpp discovery and capability resolution prefer runtime n_ctx and fall back to n_ctx_train only when runtime context is unavailable.
• llama.cpp /props parsing supports both current top-level default_generation_settings.n_ctx and the older nested params.n_ctx shape.
• Existing strategy separation is preserved: vLLM and llama.cpp still parse capabilities through their own backend-specific paths.

Tests

• dotnet test "src/Netclaw.Daemon.Tests/Netclaw.Daemon.Tests.csproj" --filter "FullyQualifiedName~OpenAiCompatibleCapabilityResolverTests|FullyQualifiedName~LlamaCppBackendStrategyTests"
• dotnet test "src/Netclaw.Configuration.Tests/Netclaw.Configuration.Tests.csproj" --filter "FullyQualifiedName~ProbeHelpersTests"
• pwsh "./scripts/Add-FileHeaders.ps1" -Verify
• git diff --check