Validate function tag literals before execution

[AkashKumar7902]

Summary

• Validate literal function.tag values against image tag syntax before constructing the function image reference.
• Preserve semver constraint resolution and valid non-semver literal tags such as hash-style build tags.
• Add coverage for invalid literal tags that previously reached runtime execution.

Motivation

Fixes #4500.

Invalid tag values that were neither semantic versions nor semantic version constraints could be appended to the image name and only fail later during function execution. This catches those values during tag resolution with an actionable error.

Validation

• go test ./pkg/fn/runtime
• go test ./pkg/fn/...
• go test -cover ./pkg/fn/runtime
• go vet ./pkg/fn/runtime
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.4 run ./pkg/fn/runtime

AI Assistance

I used OpenAI GPT-5 for code exploration, implementation assistance, and test drafting. I reviewed the changes and validated them locally.

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[Copilot]

Pull request overview

This PR adds early validation for literal function.tag values to ensure they conform to OCI/Docker image tag syntax, preventing invalid tags from reaching runtime execution and failing later.

Changes:

• Validate literal (non-constraint-resolved) function.tag values against an image tag regex before constructing the final image reference.
• Factor literal-tag image construction into a helper (imageWithLiteralTag) used by both semver-exact and non-semver tag paths.
• Add test coverage for invalid literal tags (including a semver-valid-but-tag-invalid case).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
pkg/fn/runtime/tag_resolution.go	Adds image-tag syntax validation for literal tags and routes literal tag resolution through a helper returning actionable errors early.
pkg/fn/runtime/tag_resolution_test.go	Adds cases asserting invalid literal tags now fail during resolution rather than later at execution time.

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