fix(ci): run influxdb3-core tests when test harness changes

[Copilot]

The workflow triggers on changes to test/**, Dockerfile.pytest, and compose.yaml but only checked for content changes, resulting in test harness modifications being skipped.

Changes:

• Harness file detection: Check for changes to test/, Dockerfile.pytest, and compose.yaml using exact matches for root-level files
• Test routing: When only harness files change, run influxdb3-core tests instead of skipping
• Test command: Added test:codeblocks:influxdb3-core to package.json
• CI setup: Added mock credentials for content/influxdb3/core/.env.test

Behavior:

# Before: harness changes trigger workflow but skip tests
CHANGED_FILES="" → should-run=false

# After: harness changes run influxdb3-core tests
CHANGED_HARNESS_FILES="test/pytest/conftest.py" → should-run=true, test-products=["influxdb3-core"]

Content changes continue to route to product-specific tests. When both content and harness files change, content-based routing takes precedence.

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