feat: add merge_strategy attribute to coder_env resource

[kacpersaw]

Summary

Add an optional merge_strategy attribute to the coder_env resource that controls how environment variables are combined when multiple coder_env resources define the same name.

Relates to coder/coder#21885

Valid values

Strategy	Behavior
replace (default)	Last value wins
append	Appends to existing value with : separator
prepend	Prepends to existing value with : separator
error	Fail the build if another coder_env defines the same name

Example

resource "coder_env" "path_tools" {
  agent_id       = coder_agent.dev.id
  name           = "PATH"
  value          = "/home/coder/tools/bin"
  merge_strategy = "append"
}

Changes

• provider/env.go — Added merge_strategy schema field with StringInSlice validation
• provider/env_test.go — Tests for default value, all valid values, and invalid rejection
• docs/resources/env.md — Updated with new attribute docs and PATH append example

Design notes

• Default replace preserves backward compatibility
• ForceNew: true — changing strategy recreates the resource
• No CRUD changes needed; the provider just stores the value in state
• Merge logic is enforced server-side by Coder's provisioner
• Separator is always : (PATH-style); future enhancement could add custom separator

[deansheather]

Implementation seems fine but I worry about how the order for prepending/appending is going to be determined if there are multiple

[kacpersaw]

Good point! The ordering for append/prepend is determined server-side in Coder's provisioner — resources are processed in alphabetical order by Terraform resource address (per the determinism fix in coder/coder). The provider itself just stores the merge_strategy value; it doesn't perform the merging.

I've added a note to the schema description to make this explicit: "When multiple resources append or prepend to the same name, they are applied in alphabetical order by Terraform resource address."