collect: filter logs by time range

[xhebox]

What problem does this PR solve?

close #436

tiup diag collect passes -f/-t time filters, but logs collected by the remote scraper were still copied as full files. For large log files, this makes collection slower and returns data outside the requested time range.

What is changed and how it works?

• Filter parsable standard and slow logs by the requested time range on the remote host before downloading them.
• Store filtered logs in an internal scraper temporary directory and return the filtered file path in scraper output.
• Preserve the original log file path in local collection results while downloading the filtered file content.
• Keep unparseable logs unchanged instead of dropping them.
• Add tests for scraper time-range filtering and log iterator start-boundary inclusion.

Check List

Tests

• Unit test

Ran:

• go test ./...

Code changes

• No exported function/method change
• No exported variable/fields change
• No interface methods change
• No persistent data change

Side effects

• No breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch

[pingcap-cla-assistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 47.28682% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 9.98%. Comparing base (82f9593) to head (c8907ce).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
scraper/log.go	52.17%	32 Missing and 23 partials ⚠️
collector/log.go	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #496      +/-   ##
=========================================
- Coverage   10.70%   9.98%   -0.72%     
=========================================
  Files         124     124              
  Lines       12591   10569    -2022     
=========================================
- Hits         1347    1055     -292     
+ Misses      11073    9314    -1759     
- Partials      171     200      +29     

Flag	Coverage Δ
unittests	9.98% <47.29%> (-0.72%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.