set_timeout/set_timeouts: validate inputs and define the connect-stage timeout budget

P2 follow-up from the review of #22 (https://github.com/api7/ngx_http_ffi_client/pull/22#issuecomment-5088042507).

client:set_timeout(t) / client:set_timeouts(connect, send, read) are wired through but their semantics are underspecified and untested.

Gaps

1. No validation.

client.set_timeouts (lib/resty/ngx_http_ffi_client.lua:467) stores whatever it is given: self.connect_timeout = connect_timeout or self.connect_timeout. A non-number reaches the FFI call and raises a cdata conversion error from inside the library; a negative number is converted into ngx_msec_t (unsigned) and silently becomes a huge timeout. On the C side ngx_http_ffi_client_ffi_set_timeouts() (src/ngx_http_ffi_client_request.c:1817) ignores any value <= 0, so set_timeout(0) is a silent no-op rather than "no timeout" or an error.

2. The connect deadline is per-stage, not one budget, and DNS is outside it.

• DNS: rctx->timeout = clcf->resolver_timeout (src/ngx_http_ffi_client_request.c:641) — bounded by the nginx resolver_timeout directive, not by connect_timeout.
• TCP connect: ngx_add_timer(c->write, op->connect_timeout) (src/ngx_http_ffi_client_request.c:321).
• TLS handshake: a fresh ngx_add_timer(c->read, op->connect_timeout) (src/ngx_http_ffi_client_request.c:1484).

So a caller asking for a 1s connect timeout can wait resolver_timeout + 1s + 1s before connect() returns. Whether that is intended (per-stage budgets, matching cosocket) or a bug (one shared deadline) needs to be decided and documented — ai-transport maps timeouts to 504, so the wall-clock a caller can observe matters.

3. No stall tests.

There is no test that actually stalls a connect, a TLS handshake, a send, or a read and asserts the timeout fires within the configured budget with the expected error string.

Proposal

• Validate arguments in set_timeouts: reject non-numbers and negatives with a clear error; define 0 (either "unlimited" or rejected, but not a silent no-op).
• Decide and document the connect-stage model: one shared connect deadline covering DNS + TCP + TLS, or explicit per-stage budgets with DNS bounded by resolver_timeout.
• Add real stall tests for connect, TLS handshake, send, and read, asserting both the elapsed budget and the error string (timeout / timed out, matching the lua-resty-http strings ai-transport maps to 504).

Acceptance

• Documented timeout contract in the README.
• Test::Nginx cases covering each stalled stage.
• Green on both parser backends.

Related: #16 (framing error paths). Part of #18.

Start with lib/resty/ngx_http_ffi_client.lua:467 and the referenced timeout paths in src/ngx_http_ffi_client_request.c, then review the README and existing Test::Nginx coverage. Define and document the timeout contract, add stalled connect, TLS, send, and read cases with elapsed-time and error assertions, and verify both parser backends are green.