refactor: extract jupyter protocol, harden error paths, dedup git ops

- code_runner: split shared Jupyter message/URL helpers into
  `_protocol.py`; surface kernel disconnects and run_code timeouts as
  ExecutionError; add gif and plotly MIME types to Result.
- capsule: introduce `_build_http_proxy_url` so HTTP proxy callers
  stop munging ws:// URLs; `proxy_url()` now returns http(s).
- _git: collapse `_run` + `_check_result` into `_run_op` across sync
  and async Git; drop unused `build_has_upstream`.
- pty: classify unknown msg_types as non-fatal error events instead
  of raising ValueError.
- files: add `Transfer-Encoding: chunked` to streaming uploads.
- ci: remove unused Woodpecker check.yml.
- tests: expand unit coverage for code_runner and capsule features.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.woodpecker/check.yml

@@ -1,28 +0,0 @@
-steps:
-  unit-tests:
-    image: ghcr.io/astral-sh/uv:python3.13-bookworm
-    when:
-      event: push
-      path:
-        - "src/**"
-        - "tests/**"
-    commands:
-      - uv sync --dev
-      - uv run pytest -m "not integration" -v
-
-  integration-tests:
-    image: ghcr.io/astral-sh/uv:python3.13-bookworm
-    when:
-      event: pull_request
-      branch:
-        - main
-        - dev
-      path:
-        - "src/**"
-        - "tests/**"
-    environment:
-      WRENN_API_KEY:
-        from_secret: WRENN_API_KEY
-    commands:
-      - uv sync --dev
-      - uv run pytest -m integration -v