chore: Gtest migration followups second pass

[godexsoft]

High Level Overview of Change

Second follow-up PR clearing review comments left open on the merged test-migration PRs
(#7136, #7133, #7046, #7317, #7292). **Stacked on top of the first follow-up PR. **

Closes 10 more review threads with code and answers 4 without a change. Also splits
Buffer.cpp's single 230-line test into per-behaviour cases, taking the suite from
867 to 887 tests.

Context of Change

base_uint.cpp readability

• 7136.10 — the two
  bool caught + try/catch blocks became
  EXPECT_THAT(lambda, ::testing::ThrowsMessage<E>(msg)). That keeps the message
  assertion the flag version was making, and gives a real failure message instead of
  Value of: caught / Actual: false. First gmock matcher in this TU, so <gmock/gmock.h>
  was added; the target already links GTest::gmock.
• 7136.9 — renamed the
  single-letter locals in TEST_F(BaseUintTest, base_uint): u→ascending,
  v→complement, w→rehashed, z→zero, t→expectedByte, d→byte,
  h→hasher, n→counter, x→xored, tmp→parsed, and the zero variants.
  Scoped to that function only — u, v and h also appear in testComparisons and
  Nonhash, where they mean different things.

IntrusiveShared.cpp style

• 7136.19 — dropped
  braces on 9 single-statement if/for bodies. .clang-tidy sets
  readability-braces-around-statements.ShortStatementLines: 2, so one-line bodies are
  allowed unbraced, and clang-format leaves them alone.
• 7136.13 — blank
  lines between TIBase's data members and its member functions.

Benchmarks

• 7317.8 — the
  auto* b = RegisterBenchmark(...) local is gone; the calls are chained like the other
  registration site 13 lines below. That removes the "can b be null?" question rather
  than answering it with an assert.
• 7317.11 —
  RunState::release() now also resets avgPayload, and carries a comment answering "why
  not assignments?": only swap-with-empty is guaranteed to hand back the capacity, and
  these pools are large.
• 7317.12 —
  src/benchmarks added to the EXCLUDE list in cmake/XrplCov.cmake. Benchmarks are not
  run by the coverage job, so their lines could only ever report as uncovered.
• 7317.25 — the
  rocksdb entry in backendConfigs() is now just type=rocksdb, with no tuning keys, so
  there is nothing left to drift. See "Before / After" below — this was worse than a
  drift risk.

libxrpl

• 7133.9 — added
  Buffer::fill(std::uint8_t) and used it at its one call site, intToVuc in
  shamap/SHAMap.cpp. Covered by three new tests.

Buffer.cpp split into per-behaviour tests

TEST_F(BufferTest, buffer) was a single ~230-line test covering construction, equality,
copy, move, slice conversion and resize/clear in one body, with the distinctions carried by
bare { ... } blocks and comments. A failure anywhere reported as "BufferTest.buffer
failed".

It is now 18 named tests (default_constructed_is_empty, move_assign_empty_to_non_empty,
assignment_from_slice, …) plus the 3 fill tests. The shared sample data and reference
buffers moved into the fixture — kData, kHalf, emptyBuffer, firstHalf,
secondHalf, whole — which is what let each test shrink to the case it names. sane()
stays a static fixture helper.

This is the #7046.2
theme applied to one file: "this should be applied more thoroughly to tests migrated to
gtest — we still have some places where there is one test that internally splits into
blocks of tests." The rest of the migrated suite still needs the same treatment.

One deliberate exception: equality_compares_contents keeps EXPECT_TRUE(a != b) rather
than EXPECT_NE, because the comparison operators are what is under test —
EXPECT_EQ/EXPECT_NE would route through the very operator== being verified. The
original carried a comment to this effect; it is now explicit about the reason.

API Impact

• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)

Buffer::fill() is a new method on xrpl::Buffer in include/xrpl/basics/Buffer.h. Purely
additive — no existing signature, layout or behaviour changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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