fix(postgresql): fail PITR destination setup

[weicao]

Summary

• check PITR WAL destination directory creation before repository access
• emit the failed destination path and return instead of continuing after mkdir -p fails
• add focused ShellSpec coverage for the setup boundary

Contract anchors

• docs/addon-continuous-backup-pitr-chain-integrity-guide.md, pit 4: continuous/PITR chains fail closed rather than silently proceeding through missing state
• docs/addon-api/09b-backup-extensions.md: restore must clearly fail when required inputs or artifact handling cannot be initialized
• docs/addon-api/09c-restore-and-rebuild.md: PITR validation includes prepareData and the actual restored result

TDD evidence

RED commit e3832306d (Bash 3.2): 13 examples, 1 failure / 3 failed assertions. When the destination parent was a file, mkdir -p failed but restore returned success, emitted no setup diagnostic, and still listed the WAL repository.

GREEN commit 1fbbc0b62463d3e12a44f81abcfca7f4337e9686:

• focused PITR restore ShellSpec on Bash 3.2: 13 examples, 0 failures
• full PostgreSQL ShellSpec on Bash 5.3: 281 examples, 0 failures
• ShellCheck error severity, Bash syntax, and diff check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,013,600 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 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (15b4f73) to head (1fbbc0b).

Files with missing lines	Patch %	Lines
...ns/postgresql/scripts-ut-spec/pitr_restore_spec.sh	0.00%	8 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           easton/pg-pitr-restore-waldump-failfast   #3398   +/-   ##
=======================================================================
  Coverage                                     0.00%   0.00%           
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  Files                                          195     195           
  Lines                                        28338   28346    +8     
=======================================================================
- Misses                                       28338   28346    +8     

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