feat: add --dry-run mode to Rust implementation

Add comprehensive dry-run functionality to the Rust implementation that allows
users to test their configuration without making any changes to CouchDB:

- Added --dry-run/-n command line flag with clap argument parsing
- Extended CommandLineArgs struct with dry_run field
- Updated bash completion script to include new flag
- Comprehensive dry-run logic throughout sync coordinator:
  - Skip database creation with informative logging
  - Skip sync metadata retrieval and use config fallback
  - Skip deleted message handling in sync mode
  - Skip message and attachment storage with detailed simulation
  - Skip sync metadata updates with summary information
- Enhanced summary output to clearly indicate dry-run vs normal mode
- Updated all tests to include new dry_run field
- Maintains all IMAP operations for realistic mail discovery testing

This brings the Rust implementation to feature parity with the Go version
for safe configuration testing as identified in ANALYSIS.md.

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

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

rust/src/cli.rs

@@ -28,6 +28,11 @@ pub fn parse_command_line() -> CommandLineArgs {
             .help("Maximum number of messages to process per mailbox per run (0 = no limit)")
             .value_parser(clap::value_parser!(u32))
             .action(ArgAction::Set))
+        .arg(Arg::new("dry-run")
+            .short('n')
+            .long("dry-run")
+            .help("Show what would be done without making any changes")
+            .action(ArgAction::SetTrue))
         .arg(Arg::new("generate-bash-completion")
             .long("generate-bash-completion")
             .help("Generate bash completion script and exit")
@@ -44,6 +49,7 @@ pub fn parse_command_line() -> CommandLineArgs {
     CommandLineArgs {
         config_path: matches.get_one::<String>("config").map(|s| s.clone()),
         max_messages: matches.get_one::<u32>("max-messages").copied(),
+        dry_run: matches.get_flag("dry-run"),
         generate_bash_completion: matches.get_flag("generate-bash-completion"),
         help: false, // Using clap's built-in help
     }
@@ -83,7 +89,7 @@ _{}_completions() {{
 
     if [[ $cur == -* ]]; then
         # Complete with available options
-        local opts="-c --config -m --max-messages -h --help --generate-bash-completion"
+        local opts="-c --config -m --max-messages -n --dry-run -h --help --generate-bash-completion"
         COMPREPLY=($(compgen -W "$opts" -- "$cur"))
         return
     fi