fix: Use a single timestamp in server_info

[snowyukitty]

server_info sampled system_clock once when constructing InfoSection::time
and again when calculating validated_ledger.age. If those samples straddled a
whole-second boundary, the exact-age assertions could fail intermittently.

This change reuses the timestamp already stored in output.info.time for the
age calculation. The clock remains system_clock by default; making it
injectable lets the tests use fixed absolute ledger times and verify that
successful requests sample it exactly once. A regression test also preserves
the existing zero clamp for future close times.

Fix #2353

Testing

• RPCServerInfoHandlerTest.*: 12 passed; repeated 100 times (1,200 executions)
• Adjacent RPC handler and ledger tests: 75 passed
• Full unit suite: 3,294 passed, 1 disabled
• GCC 15.2 Release with -Werror; affected Clang 19 translation units
• clang-format 22.1.8, clang-tidy, C++ pre-commit hooks, codespell, and Doxygen
  1.14.0