test: Improve unit test coverage for webchat pre-engagement form components [CHI-3696]

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Description

Adds validation logic and comprehensive test coverage for the pre-engagement form components introduced in the parent PR. Covers all FormInputType variants supported in webchat forms (Input, Email, Select, DependentSelect, Checkbox).

Changes

validation.ts

• Fixed validateEmailPattern to skip validation for empty values — optional email fields no longer produce errors when blank

PreEngagementFormPhase.tsx

• Added submit-time validation: all fields are validated via validateInput (using getDefaultValue for type-correct defaults per field type) before fetchAndStoreNewSession is called; aborts submission if any field has an error

PreEngagementFormPhase.test.tsx

• Added form validation test block covering all five FormInputType types, asserting fetchAndStoreNewSession is not called when:
  • A required field is empty (Input, Email, Select, DependentSelect, Checkbox)
  • An Email field has a value that doesn't match EMAIL_PATTERN

New component test files (__tests__/Input.test.tsx, Checkbox.test.tsx, Select.test.tsx, DependentSelect.test.tsx)**

• Each asserts: label renders, error message renders when error is non-null, onBlur calls handleChange with correct { name, value }
• DependentSelect additionally asserts setItemValue is called to reset the dependent select when the dependee value changes

Checklist

• Corresponding issue has been opened
• New tests added
• Feature flags added
• Strings are localized
• Tested for chat contacts
• Tested for call contacts

Other Related Issues

Parent PR: #3865 (form definitions), #3897 (form rendering)

Verification steps

Run npm test in aselo-webchat-react-app — all 314 tests pass.

AFTER YOU MERGE

1. Cut a release tag using the Github workflow. Wait for it to complete and notify in the #aselo-deploys Slack channel.
2. Comment on the ticket with the release tag version AND any additional instructions required to configure an environment to test the changes.
3. Only then move the ticket into the QA column in JIRA

You are responsible for ensuring the above steps are completed. If you move a ticket into QA without advising what version to test, the QA team will assume the latest tag has the changes. If it does not, the following confusion is on you! :-P

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