refactor(phpunit): move ConsoleOutput's warning prefix to [Gesso] with the console_report rename

[wadakatu]

背景

#504 (PR for the GESSO_* env rename) found the inconsistency it names in its
"Current behavior" section: the two env-driven resolvers write the same kind of
invalid-value fallback under different prefixes.

// src/ValidationOutput.php:50
fwrite(STDERR, "[Gesso] WARNING: Invalid GESSO_VALIDATION_FORMAT value ...");

// src/PHPUnit/ConsoleOutput.php:37
fwrite(STDERR, "[OpenAPI Coverage] WARNING: Invalid GESSO_CONSOLE_OUTPUT value ...");

#504 planned to fix it in the same change and its acceptance criteria say
ConsoleOutput.php should emit [Gesso] WARNING:. That was dropped from the
#504 PR and moved here.

問題

tests/Unit/Compatibility/DiagnosticPrefixesBaselineTest::identity_neutral_prefixes_retain_v1_9_parity
asserts that every prefix outside $brandedPrefixes ([openapi-contract-testing],
[Gesso], [Gesso deprecation]) is byte-identical between
tests/fixtures/compatibility/v1.9-diagnostic-prefixes.json and
v2-diagnostic-prefixes.json. Moving ConsoleOutput.php's two emissions out of
[OpenAPI Coverage] breaks it.

Keeping the test passing would require adding [OpenAPI Coverage] to
$brandedPrefixes — the largest neutral prefix, 35 emissions across 6 files —
which guts the guard for a cosmetic change. #504 checked docs/versioning.md:99
(where [OpenAPI Coverage] is indeed absent, so no documented contract is
touched) but not this test, which is stricter than the policy.

Doing it in a v2 minor is therefore the wrong release. In v3 the parity guard's
premise ends anyway: #502 renames the setting itself to console_report, and the
prefix can move with it.

やること

• Change both fwrite(STDERR, ...) calls in src/PHPUnit/ConsoleOutput.php to
  the [Gesso] WARNING: prefix.
• Move the src/PHPUnit/ConsoleOutput.php row from [OpenAPI Coverage] to
  [Gesso] in tests/fixtures/compatibility/v2-diagnostic-prefixes.json.
• Decide what identity_neutral_prefixes_retain_v1_9_parity asserts once v3 is
  allowed to move neutral prefixes — retire it, or re-baseline it against a v3
  fixture. Record the decision in docs/versioning.md.

ゴール

• rg '\[OpenAPI Coverage\]' src/PHPUnit/ConsoleOutput.php returns nothing.
• The diagnostic-prefix baseline tests state what they pin for v3 rather than
  being edited around.
• composer ci passes.

[wadakatu]

Precondition filed from a pre-v3 audit: #552.

tests/Unit/Compatibility/DiagnosticPrefixesBaselineTest.php:38 builds its accumulator with array_fill_keys(array_keys($fixture), []) and :53 iterates array_keys($fixture) — a prefix absent from the fixture is structurally invisible. [OpenAPI Strict Additional Properties] and [OpenAPI SDK Exercise] have both slipped through the whole of v2 as a result.

More relevant to this issue: identity_neutral_prefixes_retain_v1_9_parity (:78-93) compares the entire map including per-file counts, so adding a neutral key breaks it and a non-branded prefix string cannot move files. That decides how far this issue can relocate a literal, so it is worth settling what the parity test should pin before the console_report rename lands.