fix(postgresql): parse PITR switch interval as decimal

[weicao]

Summary

• parse validated SWITCH_WAL_INTERVAL_SECONDS explicitly as base 10 during PITR startup
• preserve the existing default for unset or malformed input
• cover leading-zero decimal intervals so Bash cannot silently shorten the WAL-switch throttle through octal interpretation

Candidate

• target branch: easton/pg-pitr-retention-ttl-base10
• exact base: cd5ef60fcffc6af5d8c6549385262f96b3973dca (PR fix(postgresql): parse PITR retention TTL as decimal #3391)
• exact head: 2865c1c85
• relevant open child PRs targeting the base: none
• other open weicao PostgreSQL PR targeting main: fix(postgresql): stop pgbouncer after setup failure #3331, excluded because it is the independent PgBouncer setup lane

Contract

• kubeblocks-addon-docs/docs/addon-continuous-backup-pitr-chain-integrity-guide.md: Continuous backup must keep advancing its recoverable WAL chain without silent scheduling distortion
• kubeblocks-addon-docs/docs/addon-api/09b-backup-extensions.md: Continuous/PITR archive production and time-range behavior must remain explainable and testable

TDD evidence

RED commit b8a04ccff:

• configured SWITCH_WAL_INTERVAL_SECONDS=00300
• elapsed time: 350 - 123 = 227s
• expected decimal behavior: remain throttled until 300s, no PostgreSQL probe, checkpoint stays 123
• observed before fix: interval remained 00300, Bash interpreted it as octal 192, probed PostgreSQL, and advanced checkpoint to 350
• focused result: 1 example, 1 failure with 2 failed assertions

GREEN exact head 2865c1c85:

• focused Bash 3.2: 61 examples, 0 failures
• full PostgreSQL Bash 5.3: 274 examples, 0 failures
• ShellCheck severity error: clean
• Bash syntax and git diff --check: clean
• Helm lint: 1 chart, 0 failures
• Helm render: 41 documents, 1,012,462 bytes

Runtime boundary

This is source-level scheduling arithmetic and ShellSpec validation only. Runtime packaging, environment execution, cleanup, and formal acceptance counts remain Test-owned and are not claimed by this PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (cd5ef60) to head (2865c1c).

Files with missing lines	Patch %	Lines
...ons/postgresql/scripts-ut-spec/pitr_backup_spec.sh	0.00%	13 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           easton/pg-pitr-retention-ttl-base10   #3392   +/-   ##
===================================================================
  Coverage                                 0.00%   0.00%           
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  Files                                      195     195           
  Lines                                    28274   28287   +13     
===================================================================
- Misses                                   28274   28287   +13     

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