Add credential test for n8n review (0.2.3) - #6

Why

The n8n Creator Portal review of 0.2.2 came back "Changes Required": missing credential test. The BatchData API credential class had no test; the check lived on the nodes as a testedBy method.

Changes

• BatchDataApi credential now has test: ICredentialTestRequest: GET https://api.batchdata.com/api/v2/property/search-sessions?take=1. The authenticate Bearer header is applied. A valid token returns 200 and a bad token returns 401. The call is read-only and does not bill property records (the same endpoint the New Search Session trigger polls). responseCode rules give clear messages for 401, 403 and 429.
• Remove the node-level testedBy: 'batchDataApiTest' methods (credentialMethods.ts, credentialTest.ts) from all three nodes, so the credential test is the single check.
• Unit tests: replace the two node-method tests with a test of the credential test definition and a test that no node still uses testedBy.
• Bump to 0.2.3 and update the changelog.

Verification

• n8n-node lint passes, and so do the build and 40/40 tests.
• The official scanner's analyzePackage (@n8n/scan-community-package 0.43.0, with inline config disabled) passes on both the source and the packed 0.2.3 tarball.
• Live run of the credential's request: the sandbox token returns 200 and a bad token returns 401 with the rule message.

After merge, tag 0.2.3 to publish with provenance.