Fix #5298: Update apply_diff tool description to mark :start_line: as optional

- Updated MultiSearchReplaceDiffStrategy.getToolDescription() to show :start_line: as (optional) instead of (required)
- Updated MultiFileSearchReplaceDiffStrategy.getToolDescription() to show :start_line: as (optional) instead of (required)
- This aligns the Human Relay Mode tool descriptions with the official Roo Code documentation
- Fixes inconsistency where tool descriptions incorrectly marked :start_line: as required when it's actually optional

Author: roomote

src/core/diff/strategies/multi-file-search-replace.ts

@@ -112,7 +112,7 @@ Parameters:
 Diff format:
 \`\`\`
 <<<<<<< SEARCH
-:start_line: (required) The line number of original content where the search block starts.
+:start_line: (optional) The line number of original content where the search block starts.
 -------
 [exact content to find including whitespace]
 =======

src/core/diff/strategies/multi-search-replace.ts

@@ -106,7 +106,7 @@ Parameters:
 Diff format:
 \`\`\`
 <<<<<<< SEARCH
-:start_line: (required) The line number of original content where the search block starts.
+:start_line: (optional) The line number of original content where the search block starts.
 -------
 [exact content to find including whitespace]
 =======