[BUG] Make file_http_server.h compile on Windows

[thc1006]

Fixes #4299
Refs #4287

The problem

FileHttpServer::FileGetSuccess() called std::replace(filename.begin(), filename.end(), ...) on a const std::string &, which yields const iterators, so the _WIN32 branch could not compile. clang 18 and gcc 12 both reject it; it is plain C++, not an MSVC quirk. It survived because no translation unit in the repository includes this installed header, so CI never compiled it on any platform.

The fix

The separator substitution runs on a local copy on Windows; the #else branch binds a reference so other platforms do not pay for a copy. <sstream> is now included directly for std::ostringstream, and <algorithm> (for the Windows-only std::replace) is included under _WIN32; both were previously resolving only because socket_tools.h happens to include them.

Regression protection (added in review)

The header still had no test compiling it, so the fix could regress unnoticed. A compile-only target (file_http_server_header_compile, a CMake OBJECT library and a Bazel cc_library) now includes the header as its only translation unit. CI compiles the POSIX branch on Linux and macOS and the _WIN32 branch on the Windows runners, so a compile-time break in the active platform branch of this header now fails the build. It is compiled, not run. It does not prove direct-include hygiene, since a symbol can still resolve through a transitive header; that is checked separately by the repository's IWYU workflow. The probe uses no symbol from the header on purpose, so its include is marked // IWYU pragma: keep.

Compiling the header for the first time under the maintainer configuration also surfaced pre-existing issues in it that no build had seen before: three extra semicolons after in-class function bodies (errors under -Wextra-semi -Werror) and two single-character string-literal searches, find(".") and find_last_of("."), that clang-tidy performance-faster-string-find flags. Both are fixed here so the header clears the same gates as the rest of the tree rather than raising a limit.

Not in this PR

The header has neighbouring robustness gaps (unchecked tellg(), ignored read() result, non-ASCII filenames, URI query handling) and a path-confinement concern already being handled privately. They are recorded in #4287 rather than folded into this scoped build fix.

Verification

• The compile-only target builds under -Wall -Wextra -Werror -Wextra-semi on clang for C++14, 17 and 20, and on the repository's Windows runners for the _WIN32 branch.
• clang-format 18 clean; the CHANGELOG entry is within the markdown line-length limit.
• Rebased onto current main, keeping both the [BUG] Make file_http_server.h compile on Windows #4300 and [CODE HEALTH] Move SDK trace and metrics test helpers into anonymous namespaces #4303 CHANGELOG entries; GitHub reports the branch mergeable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.24%. Comparing base (29d7a75) to head (4642315).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4300      +/-   ##
==========================================
- Coverage   81.25%   81.24%   -0.01%     
==========================================
  Files         446      446              
  Lines       18872    18872              
==========================================
- Hits        15332    15330       -2     
- Misses       3540     3542       +2     

see 1 file with indirect coverage changes

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[owent]

This is dead code and will be removed in #4306 . Maybe this PR can be closed.

[thc1006]

Superseded by #4306, which removes file_http_server.h entirely. That's the better call since the header is unused, so there's nothing left to fix here. Closing.

Thanks @owent for the cleaner fix and @dbarker for landing it.