Update to 1.38 of the otel genai standard convention

[stephentoub]

https://github.com/open-telemetry/semantic-conventions/releases/tag/v1.38.0

These blob, file, and uri parts just got added to the spec earlier today and were the only updates for 1.38 we didn't yet incorporate.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR updates the OpenTelemetry implementations in Microsoft.Extensions.AI to align with the Semantic Conventions for Generative AI systems v1.38. The key changes include refactoring how content parts are serialized for telemetry, introducing specific part types (blob, uri, file) to replace the generic representation, and adding a new comprehensive test to verify all official OpenTelemetry content part types are serialized correctly.

• Updates version references from v1.37 to v1.38 in documentation comments
• Introduces three new structured part types (OtelBlobPart, OtelUriPart, OtelFilePart) to replace generic serialization
• Adds DeriveModalityFromMediaType helper method to extract modality from MIME types
• Adds comprehensive test coverage for all official OpenTelemetry content part types in both streaming and non-streaming scenarios

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
OpenTelemetryChatClient.cs	Core implementation changes: adds new part types, modality derivation logic, and updates serialization to use structured formats per OpenTelemetry v1.38 spec
OpenTelemetryImageGenerator.cs	Updates version reference in documentation from v1.37 to v1.38
OpenTelemetryEmbeddingGenerator.cs	Updates version reference in documentation from v1.37 to v1.38
OpenTelemetrySpeechToTextClient.cs	Updates version reference in documentation from v1.37 to v1.38
OpenTelemetryChatClientTests.cs	Adds comprehensive test verifying all official content part types serialize correctly; updates existing test expectations for new serialization format
OpenTelemetryImageGeneratorTests.cs	Updates test expectations to match new serialization format with blob, uri, and file part types

[stephentoub]

@JamesNK, now that there's an official standard for it, it'd be cool if Aspire could render blobs/uris with appropriate media types.