Audit and complete bicepparam support across azd

## Context

Issue #2783 requested updating templates to use `.bicepparam` instead of `parameters.json`. While azd has partial `.bicepparam` support (`bicep_provider.go` detects and uses `.bicepparam` files, `local_preflight.go` supports them, tests exist in `cli_test.go`), a full audit is needed to ensure complete end-to-end support.

## Current State
- `bicep_provider.go` has a `bicepparamMode` that detects `.bicepparam` files
- `local_preflight.go` supports `.bicepparam` for validation snapshots
- Functional test `storage-bicepparam` exists
- `azd add` recognizes `main.bicepparam`

## Needed
- Audit all provisioning paths to ensure bicepparam works consistently
- Verify template init/scaffolding can generate `.bicepparam` files
- Update official templates to use `.bicepparam` where appropriate
- Document bicepparam usage

Related: #2783