fluent-bit: fix HTTP output SIGSEGV on too-small coroutine stack

fluent-bit: fix HTTP output SIGSEGV on too-small coroutine stack

Each output flush runs in a coroutine whose stack was forced to 4096 (one
page). The HTTP output plugin overflows it and crashes with SIGSEGV after
the flush (issue #30113). Pin FLB_CORO_STACK_SIZE to upstream's own 64K
floor (FLB_CORO_STACK_SIZE_MIN_BYTE), which the musl-shrunk default undercuts.

Also add a regression test: drive the dummy input into the http output and
assert fluent-bit survives a few flush cycles instead of dying from a signal
(no listener needed -- the overflow happens while composing the request).

Fixes: https://github.com/openwrt/packages/issues/30113
Signed-off-by: Alexandru Ardelean <redacted>