Fixes [BF-45702](https://jira.mongodb.org/browse/BF-45702) — `full_bazel_compiledb` failing on `enterprise-windows` and `enterprise-amazon2023-arm64` in `mongodb-mongo-master-nightly`.

1. 1. What this PR does

Wraps `ExtensionsSignaturePubKeyPathOptionTest` and its four test cases in `src/mongo/db/server_options_test.cpp` in `#ifndef MONGO_CONFIG_EXT_SIG_SECURE`.

`ServerGlobalParams::extensionsSignaturePublicKeyPath` (`server_options.h:322`) only exists when `MONGO_CONFIG_EXT_SIG_SECURE` is not defined, and the option itself is conditioned the same way in `server_options_general.idl:149-154`. The test fixture read that member unconditionally, so the translation unit did not compile in secure mode:

```
src/mongo/db/server_options_test.cpp(1606,35): error C2039:
'extensionsSignaturePublicKeyPath': is not a member of 'mongo::ServerGlobalParams'
```

1. 1. Motivation

This is a compile break, not a Windows-specific one — `MONGO_CONFIG_EXT_SIG_SECURE` is defined for every release/secure config (`src/mongo/BUILD.bazel:118-131`), which is why both a Windows and an ARM64 variant fail identically. Ordinary unittest tasks build in insecure mode, so only `full_bazel_compiledb` — which type-checks all test compilations under the release config — catches it. Regression from the flag added in `2dfabc3e52e` (SERVER-115282); `signature_validator_test.cpp` already uses this guard convention throughout.

1. 1. Reviewer reading guide

Single file, ~5 added lines. The only thing worth checking is that the `#endif` sits above `ExtensionsConfigPathOptionTest` — `extensionsConfigPath` is a different, unconditional option and must stay outside the guard.

1. 1. Followups & caveats

• No test was added for secure mode because none is possible: the option is not registered there, so the harness's `ASSERT_OK(_parser.run(...))` (line 1568) fails on the unrecognized flag before any validation runs.
• That makes the `BadValue` branch in `setExtensionsSignaturePubKeyPath` (`server_options_helpers.cpp:92-97`) *unreachable dead code* — its single caller (line 515) returns OK early because `params[...]` is always empty in secure mode. Left as-is here since removing it is a separate change; worth its own ticket.
• This does not make `full_bazel_compiledb` reliably green. That task has failed ~31% of runs over the last 30 days for unrelated reasons (memory pressure), tracked separately on the BF.