DEBUG-3498 Add exception information to the InternalErrorResponse stringification

[p-datadog]

What does this PR do?

Adds the causing exception information to InternalErrorResponse class that is used by Remote Config client.

Motivation:

When an error happens, it is currently reported as follows:

E, [2025-02-11T16:00:52.781351 #24] ERROR -- datadog: [datadog] (/usr/local/bundle/gems/datadog-2.9.0/lib/datadog/core/remote/component.rb:53:in `rescue in block in initialize') remote worker error: Datadog::Core::Remote::Client::TransportError #<Datadog::Core::Transport::InternalErrorResponse:0x00007df67ff5b128> location: /usr/local/bundle/gems/datadog-2.9.0/lib/datadog/core/remote/client.rb:34:in `sync'. reseting client state

There is no information as to the actual issue.

With this PR, the report is as follows:

W, [2025-02-13T12:46:53.334734 #645]  WARN -- datadog: [datadog] (/home/w/apps/dd-trace-rb/lib/datadog/core/diagnostics/environment_logger.rb:23:in `log_error!') DATADOG ERROR - TRACING - Agent Error: {"agent_error":"#\u003cDatadog::Core::Transport::InternalErrorResponse:0x00007d68f8d1b918\u003e, error_type:Net::ReadTimeout error:Net::ReadTimeout"}

The reason for the error is included in the end.

Change log entry
Yes: improve remote configuration diagnostic output

Additional Notes:

How to test the change?
Unit test added + I tested manually

[datadog-datadog-prod-us1]

Datadog Report

Branch report: di-rc-diag
Commit report: 15d6619
Test service: dd-trace-rb

✅ 0 Failed, 22094 Passed, 1477 Skipped, 5m 59.61s Total Time

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.65%. Comparing base (44bbcc1) to head (15d6619).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4383      +/-   ##
==========================================
- Coverage   97.66%   97.65%   -0.01%     
==========================================
  Files        1351     1351              
  Lines       82697    82701       +4     
  Branches     4198     4198              
==========================================
  Hits        80764    80764              
- Misses       1933     1937       +4     

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-13 18:25:24

Comparing candidate commit 15d6619 in PR branch di-rc-diag with baseline commit 44bbcc1 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.