http: cast input to unsigned char in ragel parsers

The HTTP ragel state machines in src/http/{chunk_parsers,request_parser,
response_parser}.rl declare obs_text = 0x80..0xFF per RFC 7230 §3.2.6,
but the generated code compares (*p) directly where p is const char*.
On platforms whose ABI defines `char` as signed (x86_64) the 0x80..0xFF
range comparisons happen to land bytes on the obs_text state by way of
how negative chars sort against the literal byte values; on platforms
where `char` is unsigned (aarch64) the same bytes route through a
"byte > 122" path and the parser rejects valid input.

Add `getkey ((unsigned char)(*p));` to each machine so ragel emits an
explicit unsigned-char cast in every comparison. The fix is local to
the three .rl files; no compiler flag or input-buffer type change is
required.

This affects both seastar's HTTP server (http_request_parser via
include/seastar/http/httpd.hh) and its HTTP client
(http_response_parser via src/http/client.cc), so the bug surfaces on
any seastar-based service running on arm64 that receives or sends
obs_text in field values.

Fixes scylladb#3413.

Author: travisdowns

src/http/chunk_parsers.rl

@@ -32,6 +32,10 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# Treat the input as unsigned so the obs_text (0x80..0xff) range is
+# accepted on platforms where `char` defaults to signed (x86_64) and
+# unsigned (aarch64) alike. See scylladb/seastar issue #3413.
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);
@@ -152,6 +156,10 @@ public:
 %%{
 
 access _fsm_;
+# Treat the input as unsigned so the obs_text (0x80..0xff) range is
+# accepted on platforms where `char` defaults to signed (x86_64) and
+# unsigned (aarch64) alike. See scylladb/seastar issue #3413.
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);

src/http/request_parser.rl

@@ -33,6 +33,10 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# Treat the input as unsigned so the obs_text (0x80..0xff) range is
+# accepted on platforms where `char` defaults to signed (x86_64) and
+# unsigned (aarch64) alike. See scylladb/seastar issue #3413.
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);

src/http/response_parser.rl

@@ -35,6 +35,10 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# Treat the input as unsigned so the obs_text (0x80..0xff) range is
+# accepted on platforms where `char` defaults to signed (x86_64) and
+# unsigned (aarch64) alike. See scylladb/seastar issue #3413.
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);