feat: add Python 3.14 support for WatsonX integration

[SyedShahmeerAli12]

ibm-watsonx-ai>=1.6.0 now supports Python 3.14. This updates the dependency pin, adds the 3.14 classifier, and switches CI to test against 3.14.

Closes #3036

[github-actions]

Hi @SyedShahmeerAli12, thanks for your interest in contributing to Haystack! 🙏

⚠️ You currently have 3 open pull requests in this repository (#3565, #3254 and this one). Our review capacity is limited, so please hold off opening more PRs until we've had a chance to review your first 2 open PRs. This helps us give each contribution the attention it deserves. Thank you!

This is an automated message to help us keep the review queue healthy.

[github-actions]

Heads-up for maintainers

This PR is from a fork and touches integrations whose integration tests require API keys.
Those tests are skipped in CI because fork PRs don't have access to repo secrets for security reasons.

Affected integrations:

• watsonx

Please run the integration tests locally (hatch run test:integration inside each folder) before approving.

[github-actions]

Coverage report (watsonx)

This PR does not seem to contain any modification to coverable code.

[sjrl]

Thanks!