Sync fork updates (incl. #1921/#1913) and Windows PDFium lifecycle fix

[Dr-Usman]

This PR proposes syncing the current fork branch (Dr-Usman:master) into upstream master.

Context:

• This branch includes prior fork updates and is currently ahead of upstream.
• It includes our Windows printing stability fix (post-print crash after Printing.layoutPdf).

Included upstream work already present in this fork:

1. PR Fix threading issues in iOS and macOS printing plugin #1921
   Link: Fix threading issues in iOS and macOS printing plugin #1921
   Title: Fix threading issues in iOS and macOS printing plugin
   Summary:

• Resolves thread/deadlock issues reported across platforms (macOS/iOS deadlocks and Windows property-related instability noted in the PR context).
• Ensures printing logic is coordinated on the main thread with a runloop-driven approach so UI remains responsive while waiting for Dart PDF data.
• Includes handling to avoid duplicated .pdf extension behavior.

2. PR Fixed deadlock on macOS & added filtering of redundant layout requests #1913
   Link: Fixed deadlock on macOS & added filtering of redundant layout requests #1913
   Title: Fixed deadlock on macOS & added filtering of redundant layout requests
   Summary:

• Replaces semaphore-based blocking with RunLoop polling to avoid deadlocks on newer Flutter merged-thread runtimes.
• Adds timeout safety during PDF wait.
• Adds layout parameter caching to avoid redundant onLayout calls.
• Shares PrintJobs between plugin instances (multi-window safety) and uses randomized job ID start values.

Additional Windows fix included in this fork:

• Move PDFium init/destroy to plugin lifetime in printing/windows/printing.cpp.
• Remove per-job FPDF_DestroyLibrary() calls in printing/windows/print_job.cpp.
• Fix classic PrintDlg cancel cleanup to release dialog handles safely (pd.hDC, pd.hDevMode, pd.hDevNames).

Why:

• Prevents plugin-level PDFium teardown during app lifetime, which can crash other PDF viewers in host apps after printing on Windows.
• Improves printing stability and cross-window correctness.