[fix](scanner) Fix incorrect _max_thread_num in scanner context when many queries are running. #41273

[zhiqiang-hhhh]

cherry pick from #41273

[zhiqiang-hhhh]

run buildall

[doris-robot]

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[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: function 'init' exceeds recommended size/complexity thresholds [readability-function-size]

Status ScannerContext::init() {
                       ^

Additional context

be/src/vec/exec/scan/scanner_context.cpp:77: 146 lines including whitespace and comments (threshold 80)

Status ScannerContext::init() {
                       ^

[zhiqiang-hhhh]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.01% (9345/25954)
Line Coverage: 27.59% (76787/278341)
Region Coverage: 26.39% (39437/149427)
Branch Coverage: 23.18% (20071/86604)
Coverage Report: http://coverage.selectdb-in.cc/coverage/a81971d7e78c159c11058a3f8e8671c88fc1d3be_a81971d7e78c159c11058a3f8e8671c88fc1d3be/report/index.html

[yiguolei]

run buildall

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.01% (9347/25954)
Line Coverage: 27.58% (76779/278353)
Region Coverage: 26.40% (39451/149438)
Branch Coverage: 23.18% (20079/86610)
Coverage Report: http://coverage.selectdb-in.cc/coverage/9bb76ac9ed804a6882650ce24e45bdbe94017c95_9bb76ac9ed804a6882650ce24e45bdbe94017c95/report/index.html