polish: ProblemDetail in inqcust + codify rulebook (#5, #6)

[a2chang]

Final cleanup-pass PR: brings the last legacy error-shape into line with
the rest of the API, deletes the dead CbsaFailureResponse record, and
codifies the seven recurring patterns from the PR #4 INQCUST review into
the rulebook.

Why

• Issue translator: tighten error-shape consistency and overflow guards #5 audit found InqcustController was the only controller still
  serializing a per-program legacy {failCode, message} record on the
  error path; every other controller (InqacccuController,
  CrecustController, CreaccController, DelcusController,
  DelaccController, UpdcustController, UpdaccController,
  DbcrfunController, XfrfunController) already uses
  ProblemDetail. That made the wire shape of error responses
  inconsistent across the API surface.
• Issue translator-rules: codify recurring auto-review patterns #6 audit found docs/translation-rules.md was missing several
  patterns that recurred across the auto-review of every program PR
  (defensive null-checks, deterministic randomness, retry off-by-one,
  empty-table → 404, control-baseline preservation, single error wire
  shape, @Valid on request bodies). Codifying these closes the
  feedback loop the issue called out.

What changed

Code

• InqcustController now returns ProblemDetail for all error paths
  (404 not-found, 503 random-retry-exhausted, 500 abend), matching
  InqacccuController line-for-line. The failCode is exposed as a
  ProblemDetail extension (pd.setProperty("failCode", ...)); the
  human-readable message goes in detail.
• CbsaFailureResponse deleted — the only caller was the controller
  above. No other production or test code referenced it.

Tests

• InqcustControllerTest (integration): not-found assertions updated
  from $.message to $.detail (the $.failCode extension is
  unchanged).
• InqcustControllerWebMvcTest: random-retry-exhausted assertion
  updated from $.message to $.detail. The 500/abend tests already
  asserted on $.detail / $.abendCode and required no change.

Docs

• docs/translation-rules.md gains §13 ("Recurring patterns from PR feat(inqcust): translate INQCUST to Java #4
  review") codifying all seven patterns with rationale and a code-shape
  example for the ProblemDetail rule, and §14 ("Translator checklist")
  — a 9-item bullet list a translator can scan before opening a program
  PR. References PR feat(inqcust): translate INQCUST to Java #4 and issues translator: tighten error-shape consistency and overflow guards #5 / translator-rules: codify recurring auto-review patterns #6 as the empirical source.
  Existing §13 ("Out of scope") is renumbered to §15.

Acceptance — Issue #5

• Single error wire shape documented (§13 item 1) and used by every
  controller — InqcustController was the last hold-out.
• CbsaApplication clamps the random-customer upper bound (audited
  and confirmed pre-existing: applicationRandom.nextLong(highest) + 1 cannot overflow for any positive long).
• Every controller @RequestBody parameter is @Valid-annotated
  (audited).
• Every public service method null-checks its arguments (audited).
• Companion rule update tracked in translator-rules: codify recurring auto-review patterns #6 — see below.

Acceptance — Issue #6

• §13 has a section per pattern (1: ProblemDetail, 2: null-checks,
  3: @Valid, 4: deterministic randomness, 5: retry off-by-one, 6:
  control-baseline, 7: empty-table → 404).
• Brief rationale + code-shape example for the ProblemDetail rule
  (the most common source of inconsistency).
• References PR feat(inqcust): translate INQCUST to Java #4 / issues translator: tighten error-shape consistency and overflow guards #5 / translator-rules: codify recurring auto-review patterns #6 as empirical source.
• No existing rules removed.
• §14 translator checklist appendix at the end of the doc with the
  seven items as bullets.

Validation

./mvnw -B verify — 205 tests pass (Testcontainers CockroachDB
24.3, full integration suite + WebMvc slices).

Closes #5
Closes #6

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