fix(validate-utils): write the error report to stderr synchronously so process.exit cannot truncate it

[hivecommons-hive]

Test Improvement

reportAndExit() in scripts/lib/validate-utils.mjs printed every error with
console.error() and then called process.exit(1). Node documents stderr
writes as asynchronous on POSIX when stderr is a pipe, and process.exit()
does not wait for pending writes, so the tail of a long validator report can be
dropped before the parent reads it. All 12 validate-*.mjs scripts reach this
path, and tests/e2e-data-fixture-validity.test.mjs compares a validator's full
piped output against a memoized baseline, so a truncated baseline reports the
lost lines as new errors.

Change

• reportAndExit() now builds each report into one string and writes it to
  stderr's file descriptor with fs.writeSync(), which is synchronous whatever
  stderr is connected to. Short writes are resumed and EAGAIN is retried.
• The warning report takes the same path, so a report carrying both severities
  cannot lose its warning half to the exit below and the two halves cannot
  interleave.

Why not process.exitCode = 1

The alternative suggested in the issue thread is not safe here, and I verified
why before discarding it: every validator prints a Validated N ... success
line immediately after its top-level reportAndExit() call
(validate-members.mjs:433, validate-architecture-assets.mjs:419, and ten
more). Letting the script fall through would print a success line after a failed
run. tests/validate-utils.test.mjs now pins that invariant, so the incorrect
fix cannot be reintroduced silently.

Tests

tests/validate-utils.test.mjs captures fs.writeSync instead of
console.error/console.warn, and adds:

• each report reaches stderr in a single write, warnings first;
• the module no longer routes its report through console;
• a partially accepted write is resumed until the report is complete;
• a write refused with EAGAIN is retried, and a non-EAGAIN failure propagates;
• a 2000-line report spawned through a real pipe arrives whole and exits 1;
• every validator that calls reportAndExit() prints a success line afterwards.

Verification at this head: npm run test:unit:coverage:check passes with 2324
tests green; scripts/lib/validate-utils.mjs and tests/validate-utils.test.mjs
are both at 100.00% lines and 100.00% regions, and the src files totals stay at
100.00/100.00. Prettier is clean on both files.

One honest caveat: the piped-report test passes against the old
console.error implementation too in this environment — the truncation is
load-dependent, which is why it surfaced in a merge_group runner and not
locally. The deterministic pins are the fd-write, resume and retry tests; the
piped test guards the end-to-end contract.

Related Issue

Closes #1286

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Human review required.

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[hivecommons-hive]

Important

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Hive will keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.