fix(postgresql): fail invalid WAL commit times

[weicao]

Summary

• reject transaction commit timestamps that date -d cannot parse during PITR WAL inspection
• emit the affected WAL name and return before comparing an indeterminate epoch or pulling later WALs
• add focused ShellSpec coverage for this archive-chain boundary

Contract anchors

• docs/addon-continuous-backup-pitr-chain-integrity-guide.md, pit 4: continuous/PITR chains fail closed rather than silently crossing uncertain segments
• docs/addon-api/09b-backup-extensions.md: restore must clearly fail when required repository artifacts cannot be validated
• docs/addon-api/09c-restore-and-rebuild.md: PITR validation includes the actual continuous chain and restored result

TDD evidence

RED commit 626efc6b2 (Bash 3.2): 11 examples, 1 failure / 3 failed assertions. A simulated parser failure for required WAL002's commit timestamp returned success, emitted no diagnostic, and continued to pull WAL003.

GREEN commit f4d6a90e8bfa93373fd733a682bfa2af8cccdad8:

• focused PITR restore ShellSpec on Bash 3.2: 11 examples, 0 failures
• full PostgreSQL ShellSpec on Bash 5.3: 279 examples, 0 failures
• ShellCheck error severity, Bash syntax, and diff check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,013,258 bytes

Scope

Source/static product change only. Runtime package, environment, execution, cleanup, and formal repeated-full acceptance remain tester-owned and are not claimed here.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (d2486d3) to head (f4d6a90).

Files with missing lines	Patch %	Lines
...ns/postgresql/scripts-ut-spec/pitr_restore_spec.sh	0.00%	9 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                             @@
##           easton/pg-pitr-restore-target-time-failfast   #3396   +/-   ##
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  Coverage                                         0.00%   0.00%           
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  Files                                              195     195           
  Lines                                            28320   28329    +9     
===========================================================================
- Misses                                           28320   28329    +9     

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