fix(test): alias the second namespace the merged test file needs

1e341d5413 fixed half of this. The slice that assembled the union dropped
`using namespace xrpl::telemetry;`, which supplied TWO names: `consensus` and
`seg`. Aliasing only the first left ConsensusSpanNames.cpp:156 --
`seg::consensus` -- unresolved, and clang, gcc and MSVC all failed there. It was
invisible in the previous round only because clang stops after 20 errors and the
50 `consensus` sites filled that budget, so the same defect had been present since
the merge rather than being introduced by the partial fix.

Why it was missed: the prefix scan behind the first fix sampled a line range that
did not contain line 156. This time every leading namespace qualifier in the file
was enumerated from comment- and string-stripped source and checked against the
names the file makes available, with the line each becomes available: attr, val,
op, part and span from the using-directive, consensus and seg from the aliases,
AvalancheState from a function-local using-declaration inside the test that uses
it, and std/xrpl needing nothing. Every one is declared before its first use, and
nothing else is qualified anywhere in the file.

The predicted hazard did not materialise. No `reference to 'attr' is ambiguous`
error appeared in any of the three compilers, so keeping xrpl::telemetry out of
scope and naming the two members explicitly was the right shape.

This also clears the misc-include-cleaner error that came with it. clang-tidy
reported SpanNames.h as not used directly and its exported fix deleted the
include; that fix was wrong. `seg` is declared at SpanNames.h:103, so the alias
makes the include genuinely used and the diagnostic goes away rather than needing
the include removed.

Verification: compiled. `c++ -fsyntax-only` with this file's real flags from its
compile_commands.json entry exits 0. Proven non-vacuous by removing the alias
again and reproducing CI's exact message -- "'seg' was not declared in this
scope; did you mean 'xrpl::telemetry::seg'?" -- then restoring it and returning to
exit 0. Braces balance 27/27 on stripped source, 17 TEST cases intact, pre-commit
clean including clang-format and clang-tidy. The tests still have not RUN: this is
a syntax-only check of one translation unit, so nothing linked and no assertion
executed.

src/tests/libxrpl/telemetry/ConsensusSpanNames.cpp

@@ -56,7 +56,10 @@ using namespace xrpl::telemetry::consensus::span;
 // pull xrpl::telemetry::attr (SpanNames.h:117) into scope alongside
 // consensus::span::attr and make every bare `attr::` in the phase-span tests
 // ambiguous.
+// `seg` is the other name that directive supplied, used once for the segment
+// constant the span name is built from.
 namespace consensus = xrpl::telemetry::consensus;
+namespace seg = xrpl::telemetry::seg;
 
 TEST(ConsensusSpanNames, phase_open_start_attribute_keys)
 {