[KushagraIndurkhya]

Issue #: #9261

Description of changes

When ~/.aws/config contains duplicated profile sections, the parser caught configparser.DuplicateSectionError and re-raised a generic ConfigParseError(path=path) producing Unable to parse config file: <path> without indicating which profile was duplicated, the line numbers, or what went wrong.

This change:

1. Enhances ConfigParseError to accept an optional message string rendered as:
   Unable to parse config file: <path> - <message> (preserving backwards compatibility and pickle roundtripping).
2. Updates raw_config_parse to catch configparser.DuplicateSectionError, determine the duplicated section/profile name, and locate line numbers for both the first occurrence and the duplicate occurrence:
   Found duplicate profile 'xyz' (first seen on line 5, duplicated on line 20)
3. Adds unit tests in tests/unit/botocore/test_configloader.py covering standard profiles, quoted profiles, default profile, and non-profile sections.
4. Adds tests in tests/unit/botocore/test_exceptions.py verifying message formatting and pickle roundtrip compatibility.
5. Adds a changelog entry at .changes/next-release/config-duplicate-profile-error.json.

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