feat(tapd): accept local image paths in image_url and auto-upload to TAPD image hosting

[RaphLorr]

Summary

• image_url / image_urls / media previously required an already-public URL, so embedding a local screenshot meant standing up an external image host (e.g. an object-storage bucket with public read). That pushes screenshots outside the workspace's own permission model.
• These parameters now also accept an absolute path to a local image file. The file is uploaded to TAPD's own image hosting via POST /files/upload_image, and the returned image_src is embedded instead.
• Public URLs and paths already under /tfl/ are passed through untouched, so this is backward compatible.

Changes

• tapd.py: new TAPDClient.upload_image(). It uses multipart/form-data and cannot reuse _make_request(), which sends json= and pins Content-Type: application/json — that header prevents requests from generating a multipart boundary, so it is explicitly dropped here.
• server.py: _is_local_image_path() and _upload_local_images(), hooked into _render_rich_description(). All six write tools inherit the behaviour: create_story_or_task, update_story_or_task, create_bug, update_bug, create_comments, update_comments.
• server.py: a standalone upload_image tool, for callers that want the image path / HTML snippet before composing the description themselves.
• Docstrings of the five tools that document media parameters now state that a local absolute path is accepted, since the docstring is the only description the model sees.

Behaviour notes

• Limits follow the API: 5MB per image, PNG/GIF/JPG/JPEG/BMP, one image per call.
• Requires the MCP server to run on the same machine as the image file. This always holds in stdio mode; the docstring calls it out for streamable-http deployments.
• Upload failures raise rather than degrading to a broken <img>. A ticket created with its screenshot silently missing is worse than a failed call the caller can see and retry.
• Video handling is unchanged: TAPD has no video upload endpoint, so video_url / video_urls still require an externally reachable URL.

Test plan

• Unit tests for path detection and upload/replace logic: public URL passthrough, /tfl/ paths not re-uploaded, video items skipped, missing workspace_id passthrough, upload failure raises, missing image_src raises, oversize raises, uppercase extensions accepted
• End-to-end against a real workspace: create_bug with image_url set to a local absolute path produced <img src="/tfl/pictures/202608/api_*.png" ... /> in the description; verified by reading the bug back through the API
• Confirmed POST /files/upload_image works on api.tapd.cn (the documentation page describes it on a different host)
• Verify tool descriptions render correctly via MCP inspector