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 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.81%. Comparing base (449a93a) to head (a30f3ca).
⚠️ Report is 51 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_sqlite.cc	37.50%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62673      +/-   ##
==========================================
+ Coverage   89.78%   89.81%   +0.02%     
==========================================
  Files         697      699       +2     
  Lines      215749   216373     +624     
  Branches    41294    41364      +70     
==========================================
+ Hits       193705   194327     +622     
- Misses      14124    14145      +21     
+ Partials     7920     7901      -19     

Files with missing lines	Coverage Δ
src/node_sqlite.cc	80.54% <37.50%> (-0.27%)	⬇️

... and 61 files with indirect coverage changes

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