fix(postgresql): fail invalid PITR restore times

[weicao]

Summary

• reject restore target times that date -d cannot parse
• emit a phase-specific diagnostic and return before destination setup or WAL repository access
• add focused ShellSpec coverage for the fail-fast boundary

Contract anchors

• docs/addon-continuous-backup-pitr-chain-integrity-guide.md, pit 4: continuous/PITR chains fail closed rather than silently crossing gaps
• docs/addon-api/09b-backup-extensions.md: restore must clearly fail when required inputs or repository artifacts are invalid
• docs/addon-api/09c-restore-and-rebuild.md: PITR validation includes restore-time parameter propagation and the actual continuous chain

TDD evidence

RED commit 481ba4e3b (Bash 3.2): 10 examples, 1 failure / 3 failed assertions. A simulated rc45 parsing invalid-target returned success, emitted no target-time diagnostic, and still listed the WAL repository.

GREEN commit d2486d384e0d626226bcd664b333d30401a87b56:

• focused PITR restore ShellSpec on Bash 3.2: 10 examples, 0 failures
• full PostgreSQL ShellSpec on Bash 5.3: 278 examples, 0 failures
• ShellCheck error severity, Bash syntax, and diff check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,013,091 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 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (7fe7358) to head (d2486d3).

Files with missing lines	Patch %	Lines
...ns/postgresql/scripts-ut-spec/pitr_restore_spec.sh	0.00%	7 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           easton/pg-pitr-restore-pull-failfast   #3395   +/-   ##
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  Coverage                                  0.00%   0.00%           
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  Files                                       195     195           
  Lines                                     28313   28320    +7     
====================================================================
- Misses                                    28313   28320    +7     

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