[Embedding V0] EmbeddingGenerationDatum + child trace node

# `EmbeddingGenerationDatum` + child trace node Part of: #5830 (V0 embedding generation). ## Scope Inside `ResolveEmbeddingsAsync`: ```csharp using ITrace child = trace.StartChild( "Embedding Generation", TraceComponent.Query, TraceLevel.Info); child.AddDatum("Embedding Generation Stats", new EmbeddingGenerationDatum(...)); ``` ## Datum shape | Field | Notes | |---|---| | `int EmbeddingCount` | how many texts were sent. | | `TimeSpan Latency` | wall-clock around the single `GenerateEmbeddingsAsync` call. | | `string GeneratorTypeName` | `generator.GetType().FullName`, useful in repro tickets. | | `IReadOnlyList<string> ParameterKeys` | the `@documentdb-hybridsearchquery-embedding-N` keys, **redacted of any text**. | ## What we explicitly do NOT log - Raw input text (customer data). - Returned vectors (customer data + size). ## Trace component decision V0: reuse `TraceComponent.Query` to keep the `TraceComponent` enum surface stable. Revisit if we need separate filtering. (Note in code comment.) ## Acceptance criteria - New datum class exists with the fields above. - Wired through `ResolveEmbeddingsAsync` — unit test asserts the child trace node shows up with the expected count + non-zero latency. - Privacy review sign-off recorded on the PR before merge. - `EndToEndTraceWriterBaselineTests` baselines regenerated for hybrid-search test cases that newly contain the embedding step (see #5837). ## Files likely touched - `Microsoft.Azure.Cosmos/src/Diagnostics/EmbeddingGenerationDatum.cs` (new) - `Microsoft.Azure.Cosmos/src/Tracing/TraceDatum.cs` and visitors (if a new visitor method is required for serialization to JSON / text traces) - `Microsoft.Azure.Cosmos/src/Query/Core/EmbeddingGenerationHelper.cs` (call site) ## Dependencies - `resolve-embeddings` (#5833) — datum is emitted from inside the helper.