Fix RET file navigation in tool output blocks

[dnouri]

What changed for users

This PR fixes incorrect RET file navigation from tool output in the chat buffer.

✅ Fixed behaviors

• read output now navigates to the correct file line in both collapsed and expanded views, including output that contains blank lines.
• RET on non-content collapsed lines (closing fence and ... (N more lines)) now reports a clear error instead of jumping to line 1.
• edit diff navigation now supports added (+), removed (-), and context ( ) rows.
• Generic/custom tool output with :path now maps RET navigation correctly in expanded and collapsed views.
• Generic/custom tool output with :path now uses extension-based syntax fences (same behavior as built-in file tools).
• :offset is applied only to read (as intended) and ignored for write.

Docs and tests

• Updated README wording for RET behavior in tool blocks.
• Consolidated duplicated visit-file test scaffolding into shared helpers.
• Added coverage for C-u RET inversion (same-window open path).

Verification

• Full project checks pass via pre-commit and make check:
  • byte-compile ✅
  • checkdoc ✅
  • package-lint ✅
  • unit tests: 627/627 passing ✅