Fix credential collision when connecting two Server/Data Center sites with the same PAT username

[luisdat]

Summary

saveDetailsForSite in src/atlclients/loginManager.ts computed the stored credential ID's siteId as site.product.key (e.g. "jira") for PAT (Personal Access Token) logins, instead of the site host — unlike Basic Auth, which already used the host:

const siteId = isBasicAuthInfo(credentials) ? baseLinkUrl : site.product.key;

Two different Jira or Bitbucket Server/Data Center instances authenticated via PAT with the same username therefore hash to the identical credential ID (CredentialManager.generateCredentialId is md5(siteId::username)), so connecting a second such site silently overwrites the first site's stored token — both in the in-memory credential store and in VS Code SecretStorage. The first site then fails auth on its next request and gets marked invalid/disconnected.

This is easy to hit in practice: any organization running multiple on-prem Jira/Bitbucket instances where users authenticate with the same corporate/LDAP username on each.

saveDetailsForSite is only ever used for Server/Data Center (non-cloud) sites, so the fix always scopes the credential ID to the site's host, matching the existing Basic Auth behavior:

const siteId = baseLinkUrl;

The OAuth/cloud shared-credential-per-account behavior (a separate, deliberate design for cloud accounts with multiple accessible resources) lives elsewhere and is untouched by this change.

Test plan

• Added a regression test in src/atlclients/loginManager.test.ts verifying two different Server/DC hosts sharing the same PAT username now produce distinct credential IDs.
• npm test -- src/atlclients/loginManager.test.ts src/atlclients/authStore.test.ts passes.
• Manually verified: connected two separate Jira Data Center instances via PAT with the same username; both now stay connected simultaneously (previously the first would disconnect with an auth error as soon as the second was added).

(Reopened as a fresh PR to replace #1932, which had a CLA-check issue.)

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