Add an optional lw_shared_ptr<task_context> field on seastar::task
that propagates automatically through the task chain, gated by the
SEASTAR_TASK_CONTEXT compile flag. task_context is a polymorphic
base (enable_lw_shared_from_this + virtual dtor) that users subclass
to attach per-chain state such as tracing spans. The field adds
8 bytes to task when the flag is enabled and is compiled out
otherwise.

Propagation uses a thread-local raw pointer as the authority,
mirroring how current_scheduling_group() works:

  - Reactor loop: sets TLS from task._context before run_and_dispose,
    clears after the loop iteration.
  - Task constructor: calls inherit_task_context() which reads TLS
    and shared_from_this()'s it into the new task's _context.
  - set_current_task(): keeps TLS in sync with the current task for
    paths that bypass the reactor loop (parallel_for_each inline
    resumption, thread_context::switch_in, thread.cc blocking .get()).
  - smp: work_item uses a no_context_tag to construct its task
    without inheriting, since context cannot cross shards (lw_shared_ptr
    is not thread-safe).

The primary user-facing API is with_context(ctx, func, args...) —
a function wrapper that installs ctx as TLS for the duration of
func's synchronous execution. Inline work created by func (coroutine
promises, .then() continuations, parallel_for_each branches) inherits
ctx at construction time via TLS, and their own task._context carries
it across suspensions. The caller's task is never modified, so the
function is safe inside coroutines, long-lived driver tasks, and
anywhere else.

current_task_context() and set_current_task_context() expose the
TLS accessors publicly so users can build custom RAII or awaitable
helpers on top when ergonomics demand it.

Adds tests/unit/task_context_test.cc covering the primitive accessors,
propagation across .then() chains and coroutine suspensions, gate,
async, parallel_for_each, when_any, scheduling group switches,
as_future, try_future, maybe_yield, cross-shard non-propagation,
refcount semantics, and regression coverage for the set_current_task
and thread.cc sync points.