Fix credential form crash for array-form JSON Schema type

[elias-ba]

Description

Picking the Browserless type from the credential modal crashes the form on render, change, and save. The credential schema parser couldn't handle JSON Schema's array form for nullable fields (["string", "null"]), which Browserless uses for the Base URL field. Now it can.

The Browserless schema is also internally contradictory (Base URL is required with minLength: 1, so "null" could never validate). That's fixed upstream in OpenFn/adaptors#1659.

Closes #4647

Validation steps

With this branch, open a project's Credentials, click "Add credential", and pick Browserless. The form should render and let you save.

Additional notes for the reviewer

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[github-actions]

Reviewing the diff. The changes scope to:

1. lib/lightning/credentials/schema.ex — adds normalize_property_type/1 helper for JSON Schema array-form types
2. test/lightning/credentials/schema_test.exs — test coverage
3. CHANGELOG.md — entry

This is a pure type-normalization fix in a schema-parsing helper. No DB queries, no web-layer actions, no Repo writes — only string/atom mapping over a JSON Schema map.

Security Review ✅

• S0 (project scoping): N/A, the change is to Credentials.Schema.get_types/1, a pure JSON Schema type-normalization helper with no DB access or project-scoped query.
• S1 (authorization): N/A, no new controller actions, LiveView handle_events, or other web-layer entrypoints are introduced.
• S2 (audit trail): N/A, no Repo.insert/update/delete or config-resource mutations are added; the function only parses schema property types.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.86%. Comparing base (7ac004e) to head (52929f7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4678      +/-   ##
==========================================
+ Coverage   89.80%   89.86%   +0.05%     
==========================================
  Files         444      444              
  Lines       21771    21774       +3     
==========================================
+ Hits        19552    19567      +15     
+ Misses       2219     2207      -12     

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