fix(caldav): allow same base URL with different credentials and dedupe by full identity

[AJ0070]

What does this PR do?

This PR fixes CalDAV account installation so multiple accounts can be added when they share the same base CalDAV URL but use different credentials.

• Fixes CalDav - same caldav base URL, different login credentials do not work #28678

Changes included

• Updated CalDAV add logic to compare full credential identity (url + username + password), not URL alone.
• Added idempotent handling for exact duplicate credentials (same url, username, password) to avoid duplicate records.
• Preserved valid behavior for same URL with different credentials by allowing creation of a new credential.
• Added regression tests for both scenarios above.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
  N/A (no docs change needed; behavior bugfix only).
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

• Are there environment variables that should be set?
  CALENDSO_ENCRYPTION_KEY must be set (test uses a stubbed value).

• What are the minimal test data to have?

  • A logged-in user
  • Existing CalDAV credential with:
    • url: shared base URL
    • username/password A

• What is expected (happy path) to have (input and output)?

  • Input 1: same URL with username/password B
    • Output 1: request succeeds and creates a new credential.
  • Input 2: same URL with same username/password A
    • Output 2: request succeeds idempotently without creating a second identical credential.

• Any other important info that could help to test that PR

  • Automated regression test command used:
    corepack yarn vitest run add.test.ts
  • Lint command used on touched files:
    corepack yarn biome check add.ts add.test.ts

Checklist

[CLAassistant]

All committers have signed the CLA.

[cubic-dev-ai]

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