feat: resolve relative config paths via server-derivable fallbacks

[caterryan]

Summary

Closes #99.

All 21 plugins that support TOML configuration resolve a relative config_file_path against the PLUGIN_DIR environment variable — which the InfluxDB 3 server never sets. This breaks the API-only workflow of uploading a config via POST /api/v3/plugins/files and referencing it by the same relative path in trigger_arguments.

This PR extends the resolution in all 21 plugins with two fallbacks consulted only when PLUGIN_DIR is unset:

1. PLUGIN_DIR — unchanged, keeps highest precedence (existing behavior).
2. INFLUXDB3_PLUGIN_DIR — the documented env-var form of --plugin-dir; visible to plugin code when the operator configures the server that way (e.g. the official Docker image).
3. Path(os.environ["VIRTUAL_ENV"]).parent — the processing engine activates a venv at startup whose default location is <plugin-dir>/.venv, so its parent is the plugin directory.

Fallback candidates are accepted only if the resolved file actually exists; otherwise resolution fails with an error listing every location tried.

One commit per plugin (21 commits), identical treatment adapted to each plugin's existing style (_resolve_path helpers in the subscriber/OPC UA plugins, inline blocks elsewhere). Two notes on the recently merged plugins:

• sagemaker already consulted INFLUXDB3_PLUGIN_DIR alongside PLUGIN_DIR; that precedence is kept and only the VIRTUAL_ENV fallback is appended.
• valuecounter previously defaulted to the current directory when PLUGIN_DIR was unset; that default is preserved as the final fallback, and the _resolve_config signature used by its unit tests is unchanged (82/82 tests pass).

Why this is non-breaking

• Absolute paths: unchanged.
• PLUGIN_DIR set: unchanged result; fallbacks never consulted — verified by pointing PLUGIN_DIR at a different directory than --plugin-dir and observing identical resolution before/after.
• PLUGIN_DIR unset: previously always an error; the fallbacks only turn some of those errors into successes. The remaining failure case gains a clearer message.
• system_metrics keeps its original continue-without-config behavior when resolution fails (it never aborted the run, and still doesn't).

Verification

• python3 -m py_compile clean on all 21 modified files; valuecounter's unit suite passes (82/82).
• Live end-to-end test against InfluxDB 3 Core 3.9.3 with the modified downsampler, no PLUGIN_DIR exported: uploaded configs/downsampler.toml via POST /api/v3/plugins/files, created a trigger with config_file_path=configs/downsampler.toml, and observed Reading config file / Config file loaded successfully in the server log. The same setup previously failed with Failed to get PLUGIN_DIR env var.
• Resolution-precedence scenarios (flag-only, PLUGIN_DIR exported, INFLUXDB3_PLUGIN_DIR-configured server) verified live with a side-by-side probe plugin; details and observed output in Support API-only TOML config workflow: resolve relative config_file_path without requiring the operator to export PLUGIN_DIR #99.

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