sqlite: keep source database alive during backup

[mcollina]

BackupJob held a raw DatabaseSync* to the source. If the source lost its last JS reference during an in-flight backup(), GC could destroy it while the job was still pending, and Finalize() would later dereference a dangling pointer via source_->RemoveBackup(this).

Switch BackupJob::source_ to BaseObjectPtr<DatabaseSync> so the source stays alive for the lifetime of the job, and delete the job at its terminal states (previously leaked).

[nodejs-github-bot]

Review requested:

• @nodejs/sqlite

[geeksilva97]

We can add the flag at the top

// Flags: --expose_gc

Then do

Suggested change

	if (typeof global.gc === 'function') {
	for (let i = 0; i < 5; i++) {
	global.gc();
	await new Promise((resolve) => setImmediate(resolve));
	}
	}
	for (let i = 0; i < 5; i++) {
	global.gc();
	await new Promise((resolve) => setImmediate(resolve));
	}

[codecov]

Codecov Report

❌ Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.79%. Comparing base (449a93a) to head (b9c80c9).
⚠️ Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_sqlite.cc	41.66%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62673      +/-   ##
==========================================
+ Coverage   89.78%   89.79%   +0.01%     
==========================================
  Files         697      699       +2     
  Lines      215749   216245     +496     
  Branches    41294    41336      +42     
==========================================
+ Hits       193705   194187     +482     
- Misses      14124    14141      +17     
+ Partials     7920     7917       -3     

Files with missing lines	Coverage Δ
src/node_sqlite.cc	80.61% <41.66%> (-0.20%)	⬇️

... and 55 files with indirect coverage changes

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