fix(metrics): clarify chat-template fallback logging

[YibaiMeng]

What does this PR do?

When a chat template cannot be applied, say that the client tokenizes the text directly. Log this handled fallback at INFO without a traceback. Token counting, n_errors, and reports are unchanged.

Type of change

• Bug fix

Testing

• Existing token-metrics unit tests: 62 passed.
• uv run pre-commit run --all-files passed.

[github-actions]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.34328% with 25 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@ad85144). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...utils/services/metrics_aggregator/token_metrics.py	80.95%	8 Missing ⚠️
..._utils/services/metrics_aggregator/tokenization.py	79.48%	8 Missing ⚠️
...sync_utils/services/metrics_aggregator/registry.py	61.11%	7 Missing ⚠️
...utils/services/metrics_aggregator/metrics_table.py	83.33%	1 Missing ⚠️
src/inference_endpoint/metrics/report.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #509   +/-   ##
=======================================
  Coverage        ?   80.89%           
=======================================
  Files           ?      154           
  Lines           ?    21323           
  Branches        ?        0           
=======================================
  Hits            ?    17250           
  Misses          ?     4073           
  Partials        ?        0           

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