## Description
Implements the server-side merge logic for the `merge_strategy`
attribute added to `coder_env` in [terraform-provider-coder
v2.15.0](https://github.com/coder/terraform-provider-coder/pull/489).
This allows template authors to control how duplicate environment
variable names are combined across multiple `coder_env` resources.
Relates to https://github.com/coder/coder/issues/21885
## Supported strategies
| Strategy | Behavior |
|----------|----------|
| `replace` (default) | Last value wins — backward compatible |
| `append` | Joins values with `:` separator (e.g. PATH additions) |
| `prepend` | Prepends value with `:` separator |
| `error` | Fails the build if the variable is already defined |
## Example
```hcl
resource "coder_env" "path_tools" {
agent_id = coder_agent.dev.id
name = "PATH"
value = "/home/coder/tools/bin"
merge_strategy = "append"
}
```
## Changes
- **Proto**: Added `merge_strategy` field to `Env` message in
`provisioner.proto`
- **State reader**: Updated `agentEnvAttributes` struct and proto
construction in `resources.go`
- **Merge logic**: Added `mergeExtraEnvs()` function in
`provisionerdserver.go` with strategy-aware merging for both agent envs
and devcontainer subagent envs
- **Tests**: 15 unit tests covering all strategies, edge cases (empty
values, mixed strategies, multiple appends)
- **Dependency**: Bumped `terraform-provider-coder` v2.14.0 → v2.15.0
- **Fixtures**: Updated `duplicate-env-keys` test fixtures and golden
files
## Ordering
When multiple resources `append` or `prepend` to the same key, they are
processed in alphabetical order by Terraform resource address (per the
determinism fix in #22706).
1e07ec49a6