fix(jfrog-oauth): fail when access_token is empty (#574)

## Summary

Fixes #72 - The `jfrog-oauth` module now fails with a clear error
message when the JFrog access token is empty, instead of silently
creating configurations with empty tokens.

## Changes

### 1. Added Precondition Validation (`main.tf`)

```hcl
lifecycle {
  precondition {
    condition     = data.coder_external_auth.jfrog.access_token != ""
    error_message = "JFrog access token is empty. Please authenticate with JFrog using external auth."
  }
}
```

This ensures the module fails at **plan time** with a clear error when
users haven't authenticated via external auth.

### 2. Replaced `main.test.ts` with `jfrog-oauth.tftest.hcl`

**Why we removed the TypeScript tests:**

The TypeScript tests used `runTerraformApply()` which runs `terraform
apply` directly. This approach **cannot mock data sources** like
`coder_external_auth`. The Coder provider returns empty strings for
tokens by default when running outside a real Coder workspace.

With our new precondition, the TypeScript tests would always fail
because:
1. `terraform apply` runs → empty `access_token` from mock provider
2. Precondition check fails → "JFrog access token is empty"
3. Test fails before any assertions run

**The solution:** Terraform's native `.tftest.hcl` format supports
`override_data` blocks that can properly mock data sources:

```hcl
override_data {
  target = data.coder_external_auth.jfrog
  values = {
    access_token = "valid-token-value"  # or "" to test failure
  }
}
```

### 3. Comprehensive Test Coverage

The new `jfrog-oauth.tftest.hcl` includes **12 tests** (up from 7):

| Test | What it validates |
|------|------------------|
| `test_required_vars` | Basic module works with required variables |
| `test_empty_access_token_fails` | **NEW:** Precondition rejects empty
tokens |
| `test_valid_access_token_succeeds` | Module works with valid token |
| `test_jfrog_url_validation` | **NEW:** URL must start with http(s)://
|
| `test_username_field_validation` | **NEW:** Must be "email" or
"username" |
| `test_with_npm_package_manager` | NPM config with scoped repos (script
content) |
| `test_configure_code_server` | **NEW:** IDE env vars created when
enabled |
| `test_go_proxy_env` | GOPROXY env value with multiple repos |
| `test_pypi_package_manager` | pip.conf with extra-index-url |
| `test_docker_package_manager` | register_docker commands for all repos
|
| `test_conda_package_manager` | .condarc channels configuration |
| `test_maven_package_manager` | settings.xml with servers and repos |

All package manager tests use `strcontains()` to verify the actual
script content matches expected configuration formats.

## Test Limitations (Acknowledged)

The tests verify **template rendering** but not **runtime execution**:

| ✅ What we test | ❌ What we don't test |
|----------------|----------------------|
| Configuration file formats | Script syntax errors at runtime |
| Variable interpolation | JFrog CLI compatibility |
| Precondition validation | Actual JFrog authentication |
| Script contains expected content | Commands execute successfully |

**Rationale:** The original TypeScript tests also only checked script
content (`toContain()`), not execution. Full execution testing would
require a mock JFrog server, which adds significant complexity for
limited benefit. The script is straightforward bash that configures
files and runs CLI commands.

## Testing

```bash
cd registry/coder/modules/jfrog-oauth
terraform test
# Success! 12 passed, 0 failed.
```

_Generated with [mux](https://github.com/coder/mux)_

Author: matifali

registry/coder/modules/jfrog-oauth/README.md

@@ -16,7 +16,7 @@ Install the JF CLI and authenticate package managers with Artifactory using OAut
 module "jfrog" {
   count          = data.coder_workspace.me.start_count
   source         = "registry.coder.com/coder/jfrog-oauth/coder"
-  version        = "1.2.3"
+  version        = "1.2.4"
   agent_id       = coder_agent.main.id
   jfrog_url      = "https://example.jfrog.io"
   username_field = "username" # If you are using GitHub to login to both Coder and Artifactory, use username_field = "username"
@@ -57,7 +57,7 @@ Configure the Python pip package manager to fetch packages from Artifactory whil
 module "jfrog" {
   count          = data.coder_workspace.me.start_count
   source         = "registry.coder.com/coder/jfrog-oauth/coder"
-  version        = "1.2.3"
+  version        = "1.2.4"
   agent_id       = coder_agent.main.id
   jfrog_url      = "https://example.jfrog.io"
   username_field = "email"