feat: add --validate option for output format validation

Add validation feature to ensure rendered template output conforms to
expected formats (JSON, YAML, or TOML) before writing to file or stdout.

## Features

- Add `--validate <FORMAT>` CLI option (json, yaml, or toml)
- Validates output format after rendering, before output
- Returns error code 1 on validation failure
- Silent on success (errors only), no unnecessary output messages

## Implementation

### New Modules
- Create src/validator.rs with validation logic
  - `validate_json()` - Parse and validate JSON syntax
  - `validate_yaml()` - Parse and validate YAML syntax
  - `validate_toml()` - Parse and validate TOML syntax
  - Detailed error messages with common mistake hints

### CLI Changes
- Add `ValidateFormat` enum (Json, Yaml, Toml) to cli.rs
- Add `--validate` argument using clap's ValueEnum

### Core Integration
- Update `render_template()` signature to accept `Option<ValidateFormat>`
- Validate output after rendering, before writing to file/stdout
- Update all test files to pass `None` for validate parameter

### Testing
- Add tests/test_validation.rs with 10 integration tests
  - Valid/invalid cases for each format
  - Output preservation tests
  - File output with validation
  - Default behavior without --validate flag
- Add 20+ unit tests in validator.rs module
- Add tempfile dev dependency for integration tests

## Documentation

- Update README.md with --validate option documentation
- Add usage examples for each format
- Document validation behavior (silent success, error on failure)

## Use Cases

```bash
# Validate JSON configuration
tmpltool config.json.tmpl --validate json

# Validate Kubernetes YAML manifests
tmpltool deployment.yaml.tmpl --validate yaml -o deploy.yaml

# Validate TOML build configuration
tmpltool Cargo.toml.tmpl --validate toml
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: bordeux

Cargo.lock

@@ -34,3 +34,6 @@ base64 = "0.22"
 hex = "0.4"
 bcrypt = "0.16"
 hmac = "0.12"
+
+[dev-dependencies]
+tempfile = "3.24.0"

Cargo.toml

@@ -150,6 +150,10 @@ cat template.txt | tmpltool [OPTIONS]
 - `-o, --output <FILE>` - Output file path (prints to stdout if not specified)
 - `--trust` - Trust mode: Allow filesystem functions to access absolute paths and parent directories
   - **WARNING:** Only use with trusted templates. Disables security restrictions.
+- `--validate <FORMAT>` - Validate output format (json, yaml, or toml)
+  - Validates the rendered output conforms to the specified format
+  - Exits with error code 1 if validation fails
+  - No output on success, error message only on validation failure
 
 ### Input/Output Patterns
 
@@ -181,6 +185,16 @@ cat k8s-deployment.yaml.tmpl | tmpltool | kubectl apply -f -
 
 # Trust mode for system files
 tmpltool --trust system_info.tmpl  # Can read /etc/passwd, etc.
+
+# Validate JSON output
+tmpltool config.json.tmpl --validate json
+# Exits with error if output is invalid JSON
+
+# Validate YAML output
+tmpltool k8s-deploy.yaml.tmpl --validate yaml -o deployment.yaml
+
+# Validate TOML output
+tmpltool Cargo.toml.tmpl --validate toml
 ```
 
 ## Basic Usage

README.md

@@ -1,4 +1,15 @@
-use clap::Parser;
+use clap::{Parser, ValueEnum};
+
+/// Output format for validation
+#[derive(Debug, Clone, Copy, ValueEnum)]
+pub enum ValidateFormat {
+    /// Validate as JSON
+    Json,
+    /// Validate as YAML
+    Yaml,
+    /// Validate as TOML
+    Toml,
+}
 
 /// A template rendering tool that uses Tera templates with environment variables
 #[derive(Parser, Debug)]
@@ -16,4 +27,9 @@ pub struct Cli {
     /// WARNING: This disables security restrictions. Only use with trusted templates.
     #[arg(long)]
     pub trust: bool,
+
+    /// Validate output format (json, yaml, or toml)
+    /// If validation fails, the program exits with an error and shows the validation message
+    #[arg(long, value_enum)]
+    pub validate: Option<ValidateFormat>,
 }

src/cli.rs

@@ -18,6 +18,7 @@ pub mod context;
 pub mod filters;
 pub mod functions;
 pub mod renderer;
+pub mod validator;
 
 pub use cli::Cli;
 pub use context::TemplateContext;

src/lib.rs

@@ -5,7 +5,12 @@ use tmpltool::{Cli, render_template};
 fn main() {
     let cli = Cli::parse();
 
-    if let Err(e) = render_template(cli.template.as_deref(), cli.output.as_deref(), cli.trust) {
+    if let Err(e) = render_template(
+        cli.template.as_deref(),
+        cli.output.as_deref(),
+        cli.trust,
+        cli.validate,
+    ) {
         eprintln!("Error: {}", e);
         process::exit(1);
     }

src/main.rs

@@ -1,4 +1,4 @@
-use crate::{TemplateContext, functions};
+use crate::{TemplateContext, cli::ValidateFormat, functions, validator};
 use minijinja::Environment;
 use serde::Serialize;
 use std::fs;
@@ -11,6 +11,7 @@ use std::io::{self, Read, Write};
 /// * `template_source` - Optional path to template file. If None, reads from stdin
 /// * `output_file` - Optional path to output file. If None, prints to stdout
 /// * `trust_mode` - If true, disables filesystem security restrictions
+/// * `validate_format` - Optional format to validate output against (JSON, YAML, or TOML)
 ///
 /// # Returns
 ///
@@ -19,6 +20,7 @@ pub fn render_template(
     template_source: Option<&str>,
     output_file: Option<&str>,
     trust_mode: bool,
+    validate_format: Option<ValidateFormat>,
 ) -> Result<(), Box<dyn std::error::Error>> {
     // Read template from file or stdin
     let template_content = read_template(template_source)?;
@@ -40,6 +42,11 @@ pub fn render_template(
         template_context,
     )?;
 
+    // Validate output if requested
+    if let Some(format) = validate_format {
+        validator::validate_output(&rendered, format)?;
+    }
+
     // Write output to file or stdout
     write_output(&rendered, output_file)?;