feat(telemetry): export logs over OTLP to telemetry.googleapis.com

CloudLoggingExporter is deprecated, so point ADK's Google Cloud logging
setup at the Telemetry API instead, the same way traces and metrics
already go there. Both branches it replaced are gone: the direct
WriteLogEntries client used off Agent Engine, and the structured-JSON
-to-stdout workaround used on it.

Two things had to be carried over by hand, because the OTLP mapping is
done server side and does not match what CloudLoggingExporter did:

  - MonitoredResource. CloudLoggingExporter special-cased
    `cloud.resource_id` into
    `aiplatform.googleapis.com/ReasoningEngine`. Left alone, the
    Telemetry API would detect the same resource as `generic_task` and
    silently break every existing log filter. On Agent Engine,
    GCPBatchLogRecordProcessor merges the type and its labels onto each
    record's resource. It keeps them off the resource traces and
    metrics share on purpose: the metrics pipeline reads
    `gcp.resource_type` too, and would move Agent Engine metrics off
    `prometheus_target`.

  - Log name and labels. The Telemetry API names the entry after the
    record's event name and otherwise falls back to a generic `otlp`
    log, where CloudLoggingExporter fell back to a configurable default.
    GCPBatchLogRecordProcessor stamps that default so GCP_DEFAULT_LOG_NAME
    keeps working, and re-publishes the event name as an `event.name`
    attribute so it still surfaces as a log entry label.

Two behaviour changes worth calling out. On Agent Engine the default log
name is now `aiplatform.googleapis.com/reasoning_engine_stdout` rather
than `adk-on-agent-engine`, because that is what the stdout pipeline
actually produced and what customer log filters match. And
GOOGLE_CLOUD_DEFAULT_LOG_NAME is removed rather than carried over: it
was never read by anything outside ADK.

`opentelemetry-exporter-gcp-logging` is now unused but stays declared;
dropping it churns uv.lock and five constraints files, so it is left for
a follow-up.

Co-authored-by: Max Ind <maxind@google.com>
PiperOrigin-RevId: 973832408

Author: RKest