feat(providersv2): inject static auth headers from v2 provider profiles

[Cali0707]

Summary

This PR enables injection of static provider credentials that are auth headers when providers_v2_enabled is set. It extends the existing token grant injection path to resolve and inject bearer/header credentials from provider profiles, without requiring child-env placeholder resolution.

Related Issue

Part of #896

Changes

• Gate static credential inclusion in dynamic_credentials behind providers_v2_enabled in the server
• Extend sandbox side inject_if_needed to handle static credentials as well as the existing token grant path

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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