[api][plan][python] Use request-scoped metric groups for chat token metrics

[jlon]

What

• Capture the current action metric group when a chat request is initiated.
• Record delayed chat token metrics against that captured group instead of the mutable metric group on cached chat model resources.
• Add Java and Python regression coverage for resource metric-group rebinding.

Why

Cached resources can be rebound when another action retrieves the same resource before the original request records token usage. The delayed token metrics should stay under the action that initiated the request.

Closes #859.

Tests

• mvn -pl api -DskipITs -Dspotless.check.skip=true -Dcheckstyle.skip=true -Dtest=BaseChatModelSetupTokenMetricsTest test
• mvn -pl plan -am -DskipITs -Dspotless.check.skip=true -Dcheckstyle.skip=true -Dsurefire.failIfNoSpecifiedTests=false -Dtest=ChatModelActionTest,ChatModelActionRetryTest test
• bash -lc 'export PATH="$HOME/.local/bin:$PATH"; cd python && uv run --no-sync pytest flink_agents/api/chat_models/tests/test_token_metrics.py -q'
• bash -lc 'export PATH="$HOME/.local/bin:$PATH"; cd python && uv run --no-sync ruff format --check flink_agents/api/chat_models/chat_model.py flink_agents/api/chat_models/tests/test_token_metrics.py flink_agents/plan/actions/chat_model_action.py && uv run --no-sync ruff check flink_agents/api/chat_models/chat_model.py flink_agents/api/chat_models/tests/test_token_metrics.py flink_agents/plan/actions/chat_model_action.py'
• bash -lc 'export JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64; export PATH="$JAVA_HOME/bin:$PATH"; mvn -pl api,plan -am spotless:check -Dspotless.skip=false -DskipTests -DskipITs -Dcheckstyle.skip=true'