feat: add --existing-version flag to attestation init

[migmartri]

Summary

Add a new --existing-version flag to the attestation init command that fails if the specified project version doesn't already exist in the system.

Use Case

When running packaging tasks, users want to ensure that backpatches are associated with existing versions rather than creating new ones accidentally.

Implementation

• Added require_existing_version field to AttestationServiceInitRequest proto
• Added validation in data layer that returns ErrVersionNotFound if flag is true and version doesn't exist
• Passed the flag through service, business, action, and CLI command layers
• Added CLI validation requiring --version when using --existing-version

Example Usage

$ chainloop att init --workflow sast --project my-project-2222 --replace --existing-version --version 11.11
ERR validation error: project version "11.11" not found
exit status 1

if the flag is not provided everything work as expected

$ chainloop att init --workflow sast --project my-project-2222 --replace --existing-version --version 11.11
┌───────────────────────────┬──────────────────────────────────────┐
│ Initialized At            │ 13 Nov 25 12:50 UTC                  │
├───────────────────────────┼──────────────────────────────────────┤
│ Attestation ID            │ a55db921-b6d9-4690-a634-8fe76dff172e │
│ Organization              │ john-owned                           │
│ Name                      │ sast                                 │
│ Project                   │ my-project-2222                      │
│ Version                   │ 11.11 (prerelease)                   │
│ Contract                  │ my-project-2222-sast (revision 1)    │
│ Policy violation strategy │ ADVISORY                             │
└───────────────────────────┴──────────────────────────────────────┘

and running it again makes it work

Resolves #2540