fix: Patch ux for sso start url login field

[apexDev37]

Problem

The Start URL field in the Workforce (SSO) sign-in page provides a single error message when the user's input fails validation. Error message: URLs must start with http:// or https://. Example: https://d-xxxxxxxxxx.awsapps.com/start. In some cases, this error message can be misleading or a false positive where the user's input contains a valid protocol or scheme yet fails some other form of validation required for the Start URL field.

Use case

Users who copy or retrieve their start URL from the Get credentials for [SampleRole] in the AWS Access portal for a given IAM Identity Center user will notice their start URL, for example, https://d-xxxxxxxxxx.awsapps.com/start/#, fail validation and get the following error message displayed: URLs must start with http:// or https://. Example: https://d-xxxxxxxxxx.awsapps.com/start. By default, the start URL from the Get credentials for [SampleRole] window includes a trailing forward slash and hashtag, (/#).

ℹ This differs from the start URL copied from the IAM Identity Center user Console which does not end with a trailing forward slash / or hashtag #

Example

Solution

This PR closes the following corresponding issue.

Resolves: #5462

This PR introduces patches to enhance the UX for the Start URL login field:

• Permit trailing hashtag to ensure a start URL copied by a user from the Get credentials for [SampleRole] window will not display an error message.
• Add a new error message and update validation logic to ensure input URLs with the correct protocol/scheme provide a new semantic error instead of the default.

Decisions

This section outlines notable considerations for developers/maintainers.

• [refactor] Encapsulate all error messages related to the Start URL in an enum type.

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License

License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

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This pull request modifies a feature or fixes a bug, but it does not include a changelog entry. All pull requests that introduce new features or bug fixes must have a corresponding changelog item describing the changes.