fix(otlp): return application/x-protobuf response format on otlp endpoints

[Tyagiquamar]

Summary

Fixes #6786

OTLP REST ingest endpoints (/otlp/v1/logs and /otlp/v1/traces) previously routed successful responses through into_rest_api_response, which serialized payloads to JSON and set Content-Type: application/json. OpenTelemetry exporters sending Protobuf payloads over HTTP expect Protobuf binary responses with Content-Type: application/x-protobuf per the OTLP/HTTP specification.

This PR introduces into_otlp_protobuf_response:

• Encodes successful ExportLogsServiceResponse and ExportTraceServiceResponse messages to Protobuf bytes using prost::Message::encode_to_vec().
• Sets Content-Type: application/x-protobuf with StatusCode::OK.
• Preserves standard JSON error structure when ingest errors occur.
• Updates unit tests in quickwit/quickwit-serve/src/otlp_api/rest_handler.rs to decode protobuf responses and verify the application/x-protobuf content-type header.

Test Plan

• Updated quickwit-serve OTLP unit tests (otlp_default_logs_handler, otlp_logs_handler, otlp_default_traces_handler, and otlp_ingest_traces_handler) to assert Content-Type: application/x-protobuf and decode response bodies using ExportLogsServiceResponse::decode and ExportTraceServiceResponse::decode.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Specify the success type for the error-only Result

When quickwit-serve is compiled, Err(rest_api_error) provides no information from which Rust can infer the T: Serialize parameter required by RestApiResponse::new, producing E0283. Give this result an explicit success type (for example, Result::<(), _>::Err(...)) or use a non-generic error serializer; otherwise the crate cannot build and its tests cannot run.

AGENTS.md reference: AGENTS.md:L131-L138

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[chatgpt-codex-connector]

Advertise protobuf responses in OpenAPI

When clients consume the generated OpenAPI document, all four OTLP response annotations still omit content_type, so utoipa describes their successful bodies as application/json even though this branch now returns binary protobuf; openapi.rs:89 publishes these OtlpApi paths. Update the response annotations to declare application/x-protobuf, or generated clients may attempt to decode these successful responses as JSON.

AGENTS.md reference: AGENTS.md:L23-L24

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