feat: add merge_strategy support for coder_env resources (#23107)

## 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).

site/e2e/provisionerGenerated.ts

@@ -287,6 +287,12 @@ export interface DisplayApps {
 export interface Env {
   name: string;
   value: string;
+  /**
+   * merge_strategy controls how this env var is merged when multiple
+   * coder_env resources define the same name. Valid values: "replace"
+   * (default), "append", "prepend", "error".
+   */
+  mergeStrategy: string;
 }
 
 /** Script represents a script to be run on the workspace. */
@@ -1052,6 +1058,9 @@ export const Env = {
     if (message.value !== "") {
       writer.uint32(18).string(message.value);
     }
+    if (message.mergeStrategy !== "") {
+      writer.uint32(26).string(message.mergeStrategy);
+    }
     return writer;
   },
 };