feat(agents): eject Bicep for existing Foundry projects

[hund030]

Summary

Fixes #9127. Existing Foundry projects can now eject Bicep with azd ai agent init --infra and use the same provisioning path as newly created projects without attempting to recreate the account or project.

Changes

• azure.ai.agents: eject the complete static Bicep tree for endpoint-based projects, preserve environment references, and reject unsupported networking or literal credentials before writing infra/.
• azure.ai.projects: reconcile model deployments, connections, and new or existing ACR resources against an existing project while retaining the no-work endpoint-only path.
• synthesis/templates: replace the separate brownfield template with one conditional account/project resource graph and regenerate the embedded ARM template.

Design Details

• foundryAccountName == '' selects greenfield creation; otherwise conditional existing account/project declarations target the configured brownfield resources.
• A single fully qualified modelDeployments collection works with either account path. Greenfield-only project-name validation runs in a conditional module, so existing project names are preserved.
• Provider-owned target and ACR parameters override on-disk values, secure ARM parameters are omitted from provisioning responses, and endpoint-derived canonical outputs cannot be replaced by stale or empty template outputs.
• Brownfield deployments use a separate deployment name and derive subscription/resource-group context from AZURE_AI_PROJECT_ID; endpoint-only projects still skip ARM when no reconciliation is required.

Manual Validation

• Ran azd ai agent init --infra --no-prompt against an existing Foundry project; it emitted the full static Bicep tree without modifying azure.yaml.
• Ran azd provision --preview --no-prompt from the ejected project; Bicep compiled successfully and preview reported the existing Foundry account and project as skipped.

[azure-pipelines]

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