fix: add plan-time validation to key_prefix in autolink reference resource (#3176)

[LeC-D]

Summary

Fixes #3176

Problem

terraform plan accepts any value for key_prefix in github_repository_autolink_reference, but terraform apply can fail with a 422 from the GitHub API if the value is invalid:

key_prefix must not end with a number
key_prefix must only contain letters, numbers, or .-_+=:/#

This creates a confusing plan/apply inconsistency where the plan shows a clean diff but apply fails.

Fix

Added ValidateDiagFunc to the key_prefix schema field using a regexp that mirrors GitHub's API constraints:

• Allowed characters: letters, digits, and .-_+=:/#
• Must not end with a digit

ValidateDiagFunc: validation.ToDiagFunc(validation.StringMatch(
    regexp.MustCompile(`^[a-zA-Z0-9.=+:/#_-]*[a-zA-Z.=+:/#_-]$`),
    "must only contain letters, numbers, or .-_+=:/# and must not end with a number",
)),

Both regexp and validation were already imported in this file (used by target_url_template). No new dependencies needed.

Testing

Existing acceptance tests use key_prefix values like TEST1-, TEST2-, etc. which all pass the new validation. Invalid values like PTFY25 (ends with digit) will now be caught at plan time.

[github-actions]

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