Add model validation to get_model_arn() function

[lroolle]

Problem

The get_model_arn() function in claudeb.sh lacks validation for unknown model names, which can lead to:

• Inconsistent return formats
• Silent failures when encountering unknown models
• Potential AWS Bedrock API failures
• Poor user experience with unclear error messages

Current Behavior

• Function accepts any model name without validation
• Unknown models may cause runtime errors in AWS Bedrock calls
• No guidance provided for supported model aliases

Proposed Solution

Add model validation logic with two approaches:

Option 1: Strict Validation

• Add case statement to check against known model aliases
• Exit with error for unknown models
• Display list of supported models

Option 2: Flexible Validation

• Allow custom ARNs to pass through unchanged
• Warn users about unknown model aliases
• Provide helpful error messages with suggestions

Implementation

• Add validation function for model names
• Update get_model_arn() with validation logic
• Add error handling with clear messages
• Update documentation with supported model list
• Add tests for validation scenarios

Priority

Medium - improves user experience and prevents runtime errors

Labels

• enhancement
• bug
• good first issue

Referenced from: #12 (comment)

[lroolle]

Issue resolved: Model validation already exists in claudeb.sh. Functions get_model_name() and build_model_arn() include proper validation (lines 41-44) with clear error messages for invalid model aliases and list of supported models.

[lroolle]

Reopening: I was wrong - claude.sh's get_model_arn() function lacks validation. Lines 108-111 just return unknown model aliases unchanged, unlike claudeb.sh which has proper validation. This needs to be fixed.