Fix modular downstream_component parsing for branch mapping

[fila43]

Summary

• The Jira Downstream Component Name field (customfield_10669) stores module:stream/package (e.g. postgresql:16/postgis) for modular issues, but is_modular() and parse_module_stream() expect just the package name.
• Adds extract_downstream_package() utility to version_utils.py that normalizes the raw field value: "postgresql:16/postgis" → "postgis", plain names like "curl" pass through unchanged.
• Applies the extraction in triage_agent.py where state.downstream_component is set, so determine_target_branch correctly produces stream-postgresql-16-rhel-9.8.0 instead of falling through to rhel-9.8.0.
• Updates TriageState.downstream_component field description to document the normalization.

Without this fix, modular issues get the wrong target branch in triage results and Jira comments.

Test plan

• test_extract_downstream_package — covers modular, non-modular, None, and empty inputs
• test_is_modular_requires_package_not_full_modular_string — proves raw value fails, extracted value works
• test_parse_module_stream_requires_package_not_full_modular_string — same for stream parsing
• test_map_version_to_module_branch_extracts_package_from_raw_field — end-to-end proof

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