workflow: support optional() with defaults in object type constraints

[handcaught]

Replaces typeexpr.Type() with typeexpr.TypeConstraintWithDefaults() when parsing variable, shared_variable, and output type expressions. This enables the Terraform-style optional() attribute modifier with default values inside object type constraints.

What changed:

• Changes type parsing from typeexpr.Type() to typeexpr.TypeConstraintWithDefaults() across variable, shared_variable, and output declarations.
• Adds TypeDefaults *typeexpr.Defaults to VariableNode, SharedVariableNode, and OutputNode.
• Applies type-level optional defaults before cty type conversion at compile time (for foldable values) and at runtime (for output expressions evaluated against the final run state).
• Switches shared_variable initial value type checking from strict Type().Equals() to convert.Convert(), which correctly allows optional-attribute object types.

Example:

variable "config" {
  type = object({
    name  = optional(string, "default_name")
    count = optional(number, 42)
  })
  default = {}
}

With this change, the compiled default value for config becomes {name = "default_name", count = 42} instead of failing with a type mismatch.

New tests:

• TestVariableCompile_TypeDefaults
• TestVariableCompile_TypeDefaults_MissingDefault
• TestVariableCompile_TypeDefaults_SingleArgOptional
• TestCompileSharedVariables_TypeDefaults
• TestCompileOutputs_TypeDefaults_Object

All existing tests pass. make lint-go and make lint-imports are clean.

[brokenbot]

The implementation is clean and idiomatic. Support for optional() defaults in object type constraints is correctly implemented across variables, shared variables, and outputs, both at compile-time and during runtime evaluation. Tests are thorough and cover the relevant edge cases. Approved.