feat(rpc, net): allow registry + http_router to compile under -fno-exceptions

[genisis0x]

Summary

• Closes rpc registry: support for no exceptions #2265. `registry::call` has three `try`/`catch` blocks (REPE message-based, REPE span-based, JSON-RPC) wrapping the registered handler invocation. Transitively the registry pulls in `net/http_router.hpp`, which has one `try`/`catch` plus three `throw std::runtime_error` route-misuse sites. Under `-fno-exceptions` none of that compiles, so the rpc registry could not be used in exception-free projects at all — the scenario @redrezo described in the issue.

Fix

• Guard every site with `#if __cpp_exceptions`, matching the convention already used by `glaze_exceptions.hpp`, `core_exceptions.hpp`, and `cli_menu.hpp`.

`registry.hpp` — three sites

• When exceptions are off, the `catch` is elided and the handler is invoked directly. Any exception escaping user code in that mode is UB per the `-fno-exceptions` contract; documented inline.

`http_router.hpp` — four sites

• `add(...)` try/catch: under `-fno-exceptions` the routing install is performed without try-coverage; misuse below still surfaces on stderr through the matching `#else` branches.
• Three route-misuse `throw` sites (parameter-name conflict, wildcard-not-last, wildcard-name conflict): under `-fno-exceptions` each becomes `fprintf(stderr, ...)` + `std::abort()`. Misconfiguration is still loud and fatal, which matches the existing throw behaviour for callers that did not catch.

Verification

```
clang++ -std=c++23 -fno-exceptions -Iinclude /tmp/glaze_noexc_check.cpp -o /tmp/glaze_noexc_check

(formerly: "cannot use 'try' with exceptions disabled" x4 + "cannot use 'throw' with exceptions disabled" x3)

now: clean build, exit 0

```

Default with-exceptions build: token-identical to before this commit (the `#else` branches only activate under `-fno-exceptions`). Local `jsonrpc_test` and `repe_test` rebuild and link unchanged.

Compatibility

• No public-API change.
• No behaviour change for the default build.
• New: the registry + http_router compile under `-fno-exceptions`.

[genisis0x]

Heads up on the red build (13, Debug, 23) jobs: the failure is get_name.hpp:197 [-fpermissive] 'glz_write_error' ... is used but never defined while compiling tests/networking_tests/http_router_test.cpp — same compile error that's red on the comment-only #2567 and that didn't block #2565 from merging. This PR only adds #if __cpp_exceptions guards around existing throw sites; the default-exceptions path GCC 13 takes is byte-identical to develop, so the failure isn't introduced here.

Happy to rebase onto develop after #2544 settled if that helps a re-run.