Fixes #18237

Implements the proposed solution from the issue:

When users run `terraform init` locally on a different OS/architecture
than their Coder instance, the generated .terraform.lock.hcl file may
be missing provider hashes for the target platform. This causes Terraform
to modify the lock file during provisioning and download providers from
the internet on every workspace build instead of using the shared cache,
significantly slowing down provisioning.

## Implementation

This change implements the proposed solution:
1. **Record the checksum** of .terraform.lock.hcl before running terraform init
2. **Compare checksums** after terraform init completes
3. **Warn the user** if the checksum changes, with actionable guidance

## Changes

- Added `checksumFile()` function to calculate SHA256 checksums
- Added `getTerraformLockFilePath()` helper function
- Modified `init()` to:
  - Calculate lock file checksum before running terraform init
  - Calculate lock file checksum after terraform init
  - Call `warnLockFileModified()` if checksums differ
- Created `warnLockFileModified()` that:
  - Logs a warning for observability
  - Displays a prominent, actionable warning to users with:
    - Clear explanation of the problem and its impact
    - Multi-platform solution (linux_amd64, linux_arm64, darwin_amd64, darwin_arm64, windows_amd64)
    - Single-platform quick fix option
    - Link to official documentation
- Added comprehensive tests:
  - `TestChecksumFile`: validates checksum calculation and change detection
  - `TestGetTerraformLockFilePath`: validates path construction
  - `TestWarnLockFileModified`: validates warning message content

## Advantages Over Alternative Approaches

This checksum-based approach (as proposed in the issue):
- ✅ Detects ANY lock file modification, not just specific messages
- ✅ Works regardless of Terraform version or output format changes
- ✅ Simple, reliable implementation
- ✅ No parsing of Terraform output required
- ✅ Catches edge cases where Terraform modifies lock file without the expected message

The warning message provides both:
- Multi-platform command for teams with mixed OS/arch
- Single-platform command for quick fix targeting just Coder's platform

- Use testutil.Context with WaitShort timeout instead of context.Background
- Replace slog.Make() with testutil.Logger(t)
- Use hardcoded expected checksum (0x08f39f51) instead of calling
  checksumFileCRC32 twice for comparison
- Move context and logger creation into each subtest