enhance: optimize QueryNode scheduler recovery performance from heavy load (#49825)

issue: #49707

- query node scheduler: make read tasks context-aware, use an unbuffered
add path, and cleanup canceled or near-deadline queued tasks before
queue limit checks
- query grouping: cap grouped NQ at 16 and add an NQ merge ratio guard
to avoid merging small queries into much larger groups
- REST timeout: propagate request timeout as a context deadline so
downstream query tasks receive timeout timestamps
- scheduler metrics: add ready NQ plus queue and execution duration
metrics while removing obsolete receive queue configuration
- proxy config: reduce the proxy task queue default to 256

---------

Signed-off-by: chyezh <chyezh@outlook.com>