fix(read_file): normalize empty pages string to prevent validation loop

[Gucc111]

Summary

• Normalize empty/blank pages values ("", " ") to omitted before validation in read_file, preventing invalid_tool_input rejections that lead to agent_tool_error_loop termination.
• Some models/providers emit pages: "" instead of omitting the optional field for non-PDF files; parsePdfPageRange rejects the empty string, and after 3 consecutive failures the circuit breaker kills the session.

Closes #108

Changes

src/tool/builtin/readFile.ts — added 4 lines in validateInput before the existing pages check:

if (typeof input.pages === "string" && input.pages.trim().length === 0) {
  const { pages: _pages, ...rest } = input;
  input = rest as ReadFileInput;
}

Test plan

• Verify read_file with pages: "" on a non-PDF file no longer returns invalid_tool_input
• Verify read_file with pages: "1-5" on a PDF still works correctly
• Verify tsc --noEmit passes with no type errors

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