fix(exporter/otlp/http): log error details from response on export failure

[grvmishra788]

Description

Fixes #4526.

Before: When an OTLP HTTP export fails (non-2xx response), all three exporters (trace, metrics, logs) log only resp.reason — the HTTP reason phrase (e.g. "Bad Request"). The response body is never read, so server-provided error details (e.g. a protobuf-encoded error from telemetry.googleapis.com) are silently discarded.

After: On a non-2xx response, exporters parse the body using the Content-Type header before logging:

Content-Type	Behaviour
application/x-protobuf	Deserialise as Export*ServiceResponse; extract partial_success.error_message
application/json	Parse JSON; extract partialSuccess.errorMessage or message (google.rpc.Status)
Unknown or missing	Return resp.text
Empty body or parse failure	Return resp.reason (previous behaviour, unchanged)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

The fix was verified end-to-end by constructing a 400 response with a protobuf-encoded ExportTraceServiceResponse body (containing partial_success.error_message = "Request contains too many spans (limit: 10000)") and a mocked OTLP endpoint (via the responses library). Before the fix, the logged reason was "Bad Request"; after, it is the server-provided message. The same was confirmed for a JSON google.rpc.Status body and for an empty body (fallback to resp.reason, no regression).

Run from the repo root:

pytest exporter/opentelemetry-exporter-otlp-proto-http/tests/test_response_body_parsing.py \
       exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_span_exporter.py \
       exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_log_exporter.py

• 11 unit tests in test_response_body_parsing.py — cover every branch of _parse_response_body: protobuf with/without error message, JSON partialSuccess and message fields, ; charset=utf-8 content-type variants, unknown content-type, empty body, malformed protobuf, malformed JSON.
• 3 integration tests in test_proto_span_exporter.py and test_proto_log_exporter.py — verify the parsed message appears in the error log on a real export call.
• All 41 tests pass; pylint 10.00/10.

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated