feat(validate): add custom template input validation (#565)

[ArangoGutierrez]

Summary

• Add validateCustomTemplates() to validate user-supplied custom templates before shell interpolation
• Validates: name required, no duplicates, valid phase, exactly one source (inline/file/url), HTTPS-only URLs, safe file paths (no traversal), SHA256 checksum format
• Integrates with existing ValidateTemplateInputs() pipeline
• Part of custom templates feature ([Epic] Support User-Provided Custom Templates in Holodeck Config #565), follows Task 0 API types (feat(api): add CustomTemplate type and TemplatePhase enum (#565) #701)

Test plan

• 9 new test cases covering all validation rules
• All existing validation tests continue to pass
• go build ./... passes
• go vet ./... passes

[coveralls]

Pull Request Test Coverage Report for Build 22684960905

Details

• 55 of 59 (93.22%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 48.9%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/provisioner/templates/validate.go	55	59	93.22%

Totals
Change from base Build 22684231249:	0.5%
Covered Lines:	2712
Relevant Lines:	5546

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💛 - Coveralls