fix(watsonx): use configured API endpoint

[LuisFigueroaG]

Summary

• record the Watsonx API endpoint configured by the caller instead of reporting the us-south endpoint for every request
• support both current credentials.url objects and legacy wml_credentials["url"] mappings
• omit watsonx.api_base when the SDK does not expose a trustworthy endpoint
• keep instrumentation non-intrusive when optional SDK properties are missing or raise during access

Root cause

The instrumentation always set watsonx.api_base to https://us-south.ml.cloud.ibm.com, regardless of the region or custom/on-prem endpoint used by the Watsonx client.

There was also a signature mismatch in the input handler: _handle_input declared an unused response_counter parameter, while its caller did not provide one. The instrumentation's defensive decorator swallowed that TypeError, preventing API and other input attributes from being recorded. Removing the unused parameter makes the endpoint resolution effective.

The endpoint is now resolved from explicit constructor credentials or API clients first, then from the current and legacy client shapes available on the model instance. If none provides a valid URL, no API base attribute is emitted rather than reporting incorrect telemetry.

Validation

• full Watsonx test suite: 20 passed
• focused endpoint tests: 12 passed
• Ruff lint: passed
• Python compile check: passed
• diff whitespace check: passed

Closes #2163

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