This PR adds secret-based token resolution for Hashicorp Vault authentication in TriggerAuthentication while preserving backward compatibility for existing plain-text configurations.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy (not applicable)
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added (if applicable)
• Ensure make generate-scalers-schema has been run to update any outdated generated files
• Changelog has been updated and is aligned with our changelog requirements, only when the change impacts end users
• A PR is opened to update our Helm chart (repo) (not applicable)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6026

Relates to kedacore/keda-docs#1724

What changed

• add spec.hashiCorpVault.credential.tokenFrom.secretKeyRef so Vault token auth can read the token from a Kubernetes Secret
• keep existing spec.hashiCorpVault.credential.token support for backward compatibility, but mark it deprecated via admission warnings and documentation
• resolve tokenFrom.secretKeyRef before constructing the Vault handler for both TriggerAuthentication and ClusterTriggerAuthentication
• fix the Hashicorp Vault token auth path to return a clear error when credentials are missing instead of hitting a nil reference panic
• update CRDs, generated deepcopy code, changelog, unit tests, and Hashicorp Vault e2e manifests/docs

Backward compatibility

• existing plain-text credential.token configurations continue to work
• when both tokenFrom.secretKeyRef and token are set, tokenFrom.secretKeyRef takes precedence
• users receive admission warnings recommending migration away from plain-text Vault tokens

How it was tested

• make generate
• make manifests
• make generate-scalers-schema
• PATH="$(go env GOPATH)/bin:$PATH" make golangci
• make test
• go test -tags e2e ./tests/secret-providers/hashicorp_vault -run TestDoesNotExist