Add OSS-Fuzz #508370122 reproducer to client_fuzzer corpus

Same root cause as #508342856 (fixed in 2d2efe4): an oversized
Content-Length value (here 4467440718547775) caused res.body.reserve()
to attempt a multi-petabyte allocation. The UBSAN fuzzer job surfaced
it as a std::bad_alloc-driven abort, while the ASAN job for #508342856
reported it as allocation-size-too-big. The payload_max_length_ cap
introduced in 2d2efe4 already addresses both.

test/fuzzing/corpus/clusterfuzz-testcase-minimized-client_fuzzer-6561074643271680

@@ -0,0 +1,3 @@
+ HTTP/1.1 777
+Content-Length:446744071854775
+