fix: handle ARM expression strings in preflight resource fields

[vhvb1989]

Problem

When running azd up on Bicep templates that use conditional expressions for resource fields (e.g. identity: condition ? {...} : {...}), local preflight validation fails with:

parsing bicep snapshot: json: cannot unmarshal string into Go struct field
armTemplateResource.predictedResources.identity of type bicep.armTemplateIdentity

Bicep compiles conditional expressions into ARM expression strings like "[if(equals(...))]" instead of the expected typed JSON objects. The strongly-typed Go struct fields (*armTemplateIdentity, *armTemplateSKU, etc.) cannot unmarshal these strings.

Solution

Introduce a generic armField[T] type that stores raw JSON bytes and defers parsing:

• Value() (T, bool) — typed access; returns false if the JSON is an ARM expression string
• Raw() json.RawMessage — raw JSON bytes, always works regardless of shape
• HasValue() bool — checks if the field was present and non-null

This gives callers the best of both worlds: fast typed access when the value is a real object, and graceful fallback when it's an ARM expression.

Applied to these fields in armTemplateResource:

Field	Before	After
Tags	json.RawMessage	armField[map[string]string]
SKU	*armTemplateSKU	armField[armTemplateSKU]
Plan	*armTemplatePlan	armField[armTemplatePlan]
Identity	*armTemplateIdentity	armField[armTemplateIdentity]
Copy	*armTemplateCopy	armField[armTemplateCopy]
Zones	[]string	armField[[]string]

Properties remains json.RawMessage (no single typed target), Condition remains any (already flexible).

Testing

• 7 new unit tests covering typed values, expression strings, null, absent fields, tags, and round-trip marshaling
• All 47 existing bicep package tests pass unchanged

Fixes #7154

[Copilot]

Pull request overview

Fixes local ARM preflight parsing failures when Bicep emits conditional resource fields as ARM expression strings (e.g. "[if(...)]") by deferring typed parsing and preserving raw JSON.

Changes:

• Introduced a generic armField[T] wrapper that stores raw JSON and provides typed access via Value() when possible.
• Updated armTemplateResource to use armField[...] for Tags, SKU, Plan, Identity, Copy, and Zones so expression-strings no longer break unmarshalling.
• Added unit tests validating typed values, expression strings, absent/null handling, tags, and marshal/unmarshal round-tripping.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cli/azd/pkg/infra/provisioning/bicep/local_preflight.go	Adds armField[T] and migrates several armTemplateResource fields to tolerate ARM expression strings during snapshot parsing.
cli/azd/pkg/infra/provisioning/bicep/local_preflight_test.go	Adds focused unit tests covering armField[T] behavior across typed/object values, expression strings, and basic round-trip marshaling.

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[weikanglim]

Overall, this is perfectly fine for a tactical fix to address the immediate parsing failures.

It is possible that the proposed solution isn't completely exhaustive, I left two comments on potential places to look at.

One thing to perhaps consider: It does seem that the checks are on more experimental grounds with the reliance on snapshot, and failing provision outright doesn't seem quite right. I could see this failing if it were a dedicated command like validate.