ci: update testagent version and use vcr ci mode (#14651)

## Description

Updates the testagent version to 1.34.0, which added a CI mode for the
VCR proxy feature, which throws if a cassette is missing for a given
proxied request.

## Testing

I will include a commit that removes a cassette for one of our (LLM
Observability) tests, and will attach the output from CI here. Then, the
commit will be reverted.

```
  File "/go/src/github.com/DataDog/apm-reliability/dd-trace-py/.riot/venv_py31018_14825e2ec901e2cc/lib/python3.10/site-packages/openai/_base_client.py", line 1020, in _request
    raise self._make_status_error_from_response(err.response) from None
openai.InternalServerError: Cassette openai_completions_post_076bb2cf.yaml not found while running in CI mode. Please generate the cassette locally and commit it.
```

Additionally, I verified that running the container locally did just end
up re-generating the cassette.

## Risks

None

---------

Co-authored-by: Brett Langdon <[email protected]>

Author: sabrenner

.gitlab/services.yml

@@ -12,12 +12,14 @@
       DD_REMOTE_CONFIGURATION_REFRESH_INTERVAL: 5s
       DD_DOGSTATSD_NON_LOCAL_TRAFFIC: true
   testagent:
-    name: registry.ddbuild.io/images/mirror/dd-apm-test-agent/ddapm-test-agent:v1.29.1
+    name: registry.ddbuild.io/images/mirror/dd-apm-test-agent/ddapm-test-agent:v1.34.0
     alias: testagent
     variables:
       LOG_LEVEL: ERROR
       SNAPSHOT_DIR: ${CI_PROJECT_DIR}/tests/snapshots
       SNAPSHOT_CI: 1
+      VCR_CI_MODE: true
+      OTLP_GRPC_PORT: 4136
       VCR_CASSETTES_DIRECTORY: ${CI_PROJECT_DIR}/tests/llmobs/llmobs_cassettes
       PORT: 9126
       DD_POOL_TRACE_CHECK_FAILURES: true

docker-compose.yml

@@ -121,7 +121,7 @@ services:
         volumes:
           - ddagent:/tmp/ddagent:rw
     testagent:
-        image: ghcr.io/datadog/dd-apm-test-agent/ddapm-test-agent:v1.29.1
+        image: ghcr.io/datadog/dd-apm-test-agent/ddapm-test-agent:v1.34.0
         ports:
             - "127.0.0.1:9126:8126"
         volumes: