SERVER-84018 Add multiPlannerFallbackEngaged flag to slow query log

[chriszeng1010]

Summary

• Adds a multiPlannerFallbackEngaged boolean flag to the slow query log and database profiler output
• The flag is set to true when query settings exist for a query shape but fail to produce a valid execution plan, causing the planner to fall back to multi-planning without those settings
• Does not apply to plan cached queries since they bypass the planner entirely

Changes

• op_debug.h: Added multiPlannerFallbackEngaged field directly on OpDebug
• op_debug.cpp: Emit the flag in report() (slow query log) and append() (profiler)
• get_executor_helpers.cpp: Set the flag in the NoQueryExecutionPlans catch block when query settings are removed and planning is retried
• query_settings_fallback_profiler.js: Integration test verifying the flag is present when fallback occurs and absent otherwise

Test plan

• Query with inapplicable query settings (non-existent index) triggers fallback → flag is true in profiler
• Query without query settings → flag is absent
• Functional testing via query_settings_fallback_profiler.js integration test
• Build compiles and test passes locally

[denis631]

Nice work @chriszeng1010

[kelly-cs]

Hey @chriszeng1010 - thanks for the pull request!

To consider your PR, can you sign our Contributor's Agreement? I see you already referenced the SERVER ticket, so I'll check on that

[chriszeng1010]

Signed

[denis631]

Nice work @chriszeng1010!
Thank you for your contribution and apologies for the delay!

I will run your patch on the CI and will approve as soon as it passes 💪

[denis631]

@chriszeng1010 just wanted to give you the heads up, that the PR is going through internal approval.
Also worth mentioning that currently the author is labeled as:
Author: chris zeng <chriszeng@chriss-MacBook-Air.local>

Let me know ifyou would like to change that

[chriszeng1010]

Yea got it.

Author: chris zeng <chriszeng@chriss-MacBook-Air.local>

is fine with me unless if you guys have a better recommendation