agent: Fix edit_file corrupting indentation when old_text omits first-line indent

[MartinYe1234]

The edit_file tool re-indents new_text by computing a single indent delta from the first line of old_text versus the matched buffer line, then applying that delta to every line of the replacement. When a model omits the leading indentation on only the first line of old_text/new_text (a common pattern when copying from mid-line context), the delta computed from the first line was wrongly applied to the remaining, already-correctly-indented lines, doubling their indentation.

This PR:

• Tracks the (query_row, buffer_row) line pairs aligned by the streaming fuzzy matcher so the indent of each old_text line can be compared against the buffer line it actually matched.
• Computes a separate indent delta for the lines after the first: when those lines agree on a consistent delta, it's used for the rest of the replacement; otherwise the previous uniform behavior is preserved.
• Keeps query_lines/line pairs in sync when finish() extends a match with a trailing incomplete line.
• Adds an end-to-end regression test reproducing the issue, plus unit tests for the new re-indentation logic.

Closes #60302

Release Notes:

• Fixed the agent's edit_file tool corrupting indentation when a replacement omitted leading whitespace on only its first line.