Task system: Send incoming tasks directly to waiting executors

[eddyashton]

This PR contains some follow-up cleanups from the discussion in #7269:

• Returning const std::string& from the get_name calls to avoid any additional copies. The name is generally per-type, can opt in to per-instance where appropriate, in either case it should be formatted/constructed eagerly.
• Removing the .empty() method. Replaced by a get_summary() returning some basic stats about the current queue state.
• API simplification, removing unnecessary IJobBoard and clarifying which tests (most) are testing a local JobBoard vs the global static ccf::tasks.

Also a perf improvement - rather than gluing together discrete condition_variable and locked queue (taking 2 separate mutexes in sequence) we use a condition_variable wait for workers to pause directly on the work queue.

[Copilot]

Pull Request Overview

Introduces a redesigned task execution system to route tasks directly to waiting worker threads, removes the IJobBoard interface, and centralizes delayed/periodic scheduling within JobBoard. Key changes:

• Replaces IJobBoard with concrete JobBoard offering wait-based task dispatch, delayed/periodic scheduling, and summary reporting.
• Adds ThreadManager for dynamic worker management and updates tests to use new APIs (set_worker_count, JobBoard::tick, get_summary).
• Standardizes task/action naming to return const std::string&, eliminating transient string_view returns.

Reviewed Changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/tasks/worker.h	Adds worker loop using timed waits on JobBoard.
src/tasks/thread_manager.{h,cpp}	Introduces ThreadManager for managing pooled worker threads.
src/tasks/job_board.{h,cpp}	Reimplements JobBoard with direct worker notification, delayed/periodic task handling, and summary API.
src/tasks/task_system.{h,cpp}	Simplifies global API; delegates scheduling to JobBoard and worker mgmt to ThreadManager.
src/tasks/task.h	Changes get_name() to pure virtual returning const std::string&.
src/tasks/basic_task.h	Adjusts BasicTask naming and get_name signature.
src/tasks/ordered_tasks.{h,cpp}	Updates constructor/API to use concrete JobBoard and rvalue name handling.
src/tasks/fan_in_tasks.{h,cpp}	Removes custom naming; uses concrete JobBoard.
Tests (multiple files)	Adapted to new APIs (summary checks, periodic/delayed handling moved to JobBoard instance).
Bench files	Updated task name accessors to return const std::string&.
CMakeLists.txt	Adds new source (thread_manager.cpp) and new test file (tasks_api.cpp).
.clang-tidy	Disables additional CERT concurrency checks.

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