fwserver: set AttributePathExpression when modifying schema plans

[emmayusufu]

Related Issue

Fixes #1219

Description

SchemaModifyPlan built the top-level ModifyAttributePlanRequest with AttributePath set but AttributePathExpression left as the zero value. Every nested level then derived its expression from that empty root with AtName(...), so plan modifiers received expressions missing the root step. With the schema from the issue, the modifier on nested.double_nested.str_a saw double_nested.str_a, and a modifier on a first-level nested attribute saw a single bare step.

The fix sets AttributePathExpression: path.MatchRoot(name) in both the attributes and blocks loops, which is exactly how SchemaValidate already constructs its requests. The blocks loop had the same omission, and since both child-request sites in block_plan_modification.go and attribute_plan_modification.go derive from the parent expression, fixing the root repairs expressions at every nesting depth for attributes and blocks alike.

The new test runs the issue's two-level SingleNestedAttribute shape through SchemaModifyPlan and asserts the leaf modifier's req.PathExpression. Without the fix it fails with the exact output from the report (got: double_nested.str_a). The test covers the attribute path from the report; the block loop is the same one-line fix by symmetry, and I can add a block-shaped test if you would like one.

One behavior note: providers that observed the truncated expression and matched against it will now see the corrected full expression.

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

No changes to security controls.