Fix README CI/CD badge

[austin-denoble]

Problem

Aaron called out that our badge for CI in the Python repo is currently broken: https://pinecone-io.slack.com/archives/C061PNN5GLX/p1770148171033409

Solution

The CI/CD workflows have likely been refactored and renamed since this was last updated. I think we just need to properly point at an existing workflow.

I chose Testing (main) - https://github.com/pinecone-io/pinecone-python-client/actions/workflows/on-merge.yaml, but we could use whatever makes the most sense here.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

N/A

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Note

Low Risk
Low risk documentation-only change that updates a README badge URL and does not affect runtime code or CI behavior.

Overview
Fixes the README CI badge by updating the badge image and link to point at the renamed GitHub Actions workflow (on-merge.yaml) instead of the old pr.yaml path.

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