Python: preserve ChatKit input tags in user messages

[vedanth bora (VedanthB)]

Motivation & Context

ChatKit user messages can contain UserMessageTagContent, but ThreadItemConverter.user_message_to_input only consumed text parts. Mixed-message tags were silently omitted, and a tag-only turn was discarded. This fixes the data loss reported in #8096.

Description & Review Guide

• Convert input tags through the existing tag_to_message_content extension point.
• Preserve the original order of text and non-text tag conversions by flushing buffered text around custom content.
• Keep attachments after the ordered message content, matching the existing behavior.
• Add regressions for inline text tags, tag-only messages, and custom non-text tag conversion order.

The primary review surface is ThreadItemConverter.user_message_to_input; the remaining changes are focused regression tests.

Related Issue

Fixes #8096

Contribution Checklist

• The code builds successfully.
• All new and existing unit tests pass locally.
• The change follows the repository's contribution and Python coding guidelines.
• A linked issue documents the bug and no competing issue or pull request was found.
• The change is backward compatible and preserves existing attachment ordering.

Validation performed:

• uv run poe check -P chatkit (38 tests, 100% line coverage; formatting, lint, and type checks passed)
• uv run poe build -P chatkit

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