fix(bedrock): Fix KeyError for optional 'qualifiers' in guardContent

[Ratish1]

Description

This PR fixes a KeyError: 'qualifiers' that occurred when processing Bedrock guardContent. The original bug was caused by directly accessing the qualifiers field.

The change now assumes guardContent.text is a dictionary and safely checks for the existence of the optional qualifiers key before adding it to the result.

Related Issues

Closes #959

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• [x] I ran hatch run prepare
• [x] Pre-commit hooks, formatting and linting passed locally
• [x] I ran unit tests for bedrock:
• pytest -q tests/strands/models/test_bedrock.py::test_format_request_guard_content_with_qualifiers
• pytest -q tests/strands/models/test_bedrock.py::test_format_request_guard_content_without_qualifiers

Checklist

• [x] I have read the CONTRIBUTING document
• [x] I have added any necessary tests that prove my fix is effective or my feature works
• [ ] I have updated the documentation accordingly
• [ ] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published

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