fixed asset download oom issue

[shafeeqd959]

Summary

• downloadAssets() resolved each asset's download the instant response.pipe(writeStream) was called, not when the file
  finished writing to disk. This made the downloadLimit (default 5) concurrency gate ineffective — new batches kicked off as
  fast as HTTP responses arrived, while writes queued up unbounded, exhausting the heap on large asset libraries (FATAL
  ERROR: ... JavaScript heap out of memory).
• Made onSuccess async and awaited stream.finished(assetWriterStream) (via promisify) before resolving, so each download
  promise only settles once the file write actually completes. This makes downloadLimit throttle real in-flight transfers as
  intended.
• Consolidated the separate assetWriterStream.on('error', ...) handler into the same try/catch around finished().

Testing

• Added a regression test (test/unit/export/modules/assets.test.ts) that drives onSuccess with a controllable PassThrough
  stream and asserts the download promise doesn't settle until the write stream ends — fails on pre-fix code, passes with
  the fix.
• Full contentstack-export unit suite: 483 passing, no regressions.
• tsc --noEmit clean.

[github-actions]

🔒 Security Scan Results

ℹ️ Note: Only vulnerabilities with available fixes (upgrades or patches) are counted toward thresholds.

Check Type	Count (with fixes)	Without fixes	Threshold	Result
🔴 Critical Severity	0	0	10	✅ Passed
🟠 High Severity	54	70	25	❌ Failed
🟡 Medium Severity	0	5	500	✅ Passed
🔵 Low Severity	0	0	1000	✅ Passed

⏱️ SLA Breach Summary

⚠️ Warning: The following vulnerabilities have exceeded their SLA thresholds (days since publication).

Severity	Breaches (with fixes)	Breaches (no fixes)	SLA Threshold (with/no fixes)	Status
🔴 Critical	0	0	15 / 30 days	✅ Passed
🟠 High	0	0	30 / 120 days	✅ Passed
🟡 Medium	0	1	90 / 365 days	⚠️ Warning
🔵 Low	0	0	180 / 365 days	✅ Passed

ℹ️ Vulnerabilities Without Available Fixes (Informational Only)

The following vulnerabilities were detected but do not have fixes available (no upgrade or patch). These are excluded from failure thresholds:

• Critical without fixes: 0
• High without fixes: 70
• Medium without fixes: 5
• Low without fixes: 0

❌ BUILD FAILED - Security checks failed

Please review and fix the security vulnerabilities before merging.

[github-actions]

🔒 Security Scan Results

ℹ️ Note: Only vulnerabilities with available fixes (upgrades or patches) are counted toward thresholds.

Check Type	Count (with fixes)	Without fixes	Threshold	Result
🔴 Critical Severity	0	0	10	✅ Passed
🟠 High Severity	54	70	25	❌ Failed
🟡 Medium Severity	0	5	500	✅ Passed
🔵 Low Severity	0	0	1000	✅ Passed

⏱️ SLA Breach Summary

⚠️ Warning: The following vulnerabilities have exceeded their SLA thresholds (days since publication).

Severity	Breaches (with fixes)	Breaches (no fixes)	SLA Threshold (with/no fixes)	Status
🔴 Critical	0	0	15 / 30 days	✅ Passed
🟠 High	0	0	30 / 120 days	✅ Passed
🟡 Medium	0	1	90 / 365 days	⚠️ Warning
🔵 Low	0	0	180 / 365 days	✅ Passed

ℹ️ Vulnerabilities Without Available Fixes (Informational Only)

The following vulnerabilities were detected but do not have fixes available (no upgrade or patch). These are excluded from failure thresholds:

• Critical without fixes: 0
• High without fixes: 70
• Medium without fixes: 5
• Low without fixes: 0

❌ BUILD FAILED - Security checks failed

Please review and fix the security vulnerabilities before merging.