Validate block selection bounds, trim trailing tabs, and guard poplist adjustments

[rainman74]

Motivation

• Prevent remaining Access Violations in convertall caused by invalid per-line block selection positions returned from Scintilla.
• Ensure trailing horizontal tabs are treated as removable whitespace by trimtrailingspace so trailing whitespace is fully trimmed.
• Avoid unsigned wrap/underflow when adjusting stored poplist positions after edits.

Description

• Read SCI_GETLINESELSTARTPOSITION/SCI_GETLINESELENDPOSITION into Sci_Position (ls/le) and validate against INVALID_POSITION and le < ls, neutralizing invalid lines with lps[ln]=lpe[ln]=0 so they are skipped during replacements.
• Advance block-mode input parsing with memspn in the per-line replacement loop and ensure we never pass invalid offsets to SCI_SETTARGETSTART/END/SCI_REPLACETARGET.
• Update trimtrailingspace to treat \t as whitespace (so both spaces and tabs are removed at line ends) and simplify the trimming logic accordingly.
• Harden adjustprplist to avoid unsigned underflow by checking lengths before adding or subtracting and clamping indexes when necessary.

Testing

• Inspected modified lines with nl -ba SRC/NPPTextFX.cpp | sed -n '3674,3680p;4825,4838p;4954,4961p' which showed the intended changes and succeeded.
• Verified repository changes with git diff --stat -- SRC/NPPTextFX.cpp and created a commit with git commit -m, both operations succeeded.
• Generated PR metadata and summary using the local mcp__make_pr__make_pr helper which produced the PR title/body; no Notepad++/Scintilla runtime tests were available in this environment.

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Codex Task