fix!: change UX in key assignment (backport #1998)

[mergify]

Description

Modifies the user experience (UX) of key assignment by reverting #1732. Note that this is not a full revert of #1732 because #1732 added tests that made sense to keep here as well.

Before, when a validator tried to re-use a previously assigned consumer key that they used, we would return success and the assignment would be a no-op. With this PR, we return an error instead.
The reason for this change was that we had a validator re-assigning an old consumer key they used but because the assignment was a no-op, the old consumer key was not set. As a result, the validator was confused on why the assignment was successful but the consumer key on the consumer chain remained unchanged.

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• Included the correct type prefix in the PR title
• Added ! to the type prefix if the change is state-machine breaking
• Confirmed this PR does not introduce changes requiring state migrations, OR migration code has been added to consumer and/or provider modules
• Targeted the correct branch (see PR Targeting)
• Provided a link to the relevant issue or specification
• Followed the guidelines for building SDK modules
• Included the necessary unit and integration tests
• Added a changelog entry to CHANGELOG.md
• Included comments for documenting Go code
• Updated the relevant documentation or specification
• Reviewed "Files changed" and left comments if necessary
• Confirmed all CI checks have passed
• If this PR is library API breaking, bump the go.mod version string of the repo, and follow through on a new major release

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! the type prefix if the change is state-machine breaking
• confirmed this PR does not introduce changes requiring state migrations, OR confirmed migration code has been added to consumer and/or provider modules
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage

Summary by CodeRabbit

• New Features

  • Improved key assignment security: Now triggers an error if a validator attempts to reuse the same consumer key, preventing key duplication.

• Bug Fixes

  • Enhanced error handling for scenarios where a key is already assigned, ensuring accurate failure scenarios and messages.

• Tests

  • Updated test cases and JSON structures to reflect new error handling logic for key assignments.

• Documentation

  • Updated function descriptions in test cases to specify scenarios related to key assignment failures.

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This is an automatic backport of pull request #1998 done by [Mergify](https://mergify.com).