Compile-only coverage for installed public headers

Follow-up to #4299 / #4306.

#4299 was a compile error in an installed header (file_http_server.h) that no in-repo translation unit included, so CI never compiled it on any platform. #4306 resolved that specific case by removing the unused header, but the underlying gap remains: an installed public header that nothing in the repo includes is never compiled by CI, so a break in one only surfaces when a downstream consumer builds it (and platform-specific breaks, like a Windows-only error, are especially easy to miss).

Proposal

Add a compile-only target that #includes each installed public header and is built (not run) on Linux, macOS, and Windows, so CI fails if any installed header stops compiling standalone.

A few things I'd want input on before implementing, rather than sending a large PR cold:

• Scope: start with ext/include/opentelemetry/ext/** (where #4299 was), or also cover the api/sdk/exporters installed headers?
• Build-option conditionality: many headers are gated on options (WITH_OTLP_*, etc.), so the target would compile whatever the current build config enables, matching each CI job's flags. Does that fit the existing CI matrix, or is there a preferred shape?
• Discovery: glob the installed headers at configure time, or keep an explicit list?
• Build systems: cover both CMake and Bazel, or start with one?

If this is welcome I'm happy to implement it. If there's a known reason it hasn't been done (e.g. the option-conditionality makes it awkward), I'd rather hear that first.

Start by reading #4299 and #4306, then inspect file_http_server.h, the installed headers under ext/include/opentelemetry/ext/**, and the existing CMake and Bazel CI configuration. Resolve the proposed scope, option handling, header discovery, and build-system coverage before implementing. Done means standalone compilation of the selected installed public headers is exercised across Linux, macOS, and Windows CI.