Fix: trigger result_callback for single command apps (Issue #445)

[dilanmelvin]

Summary
Fixes #445

This PR ensures that result_callback is triggered correctly even when the Typer application consists of a single command.

Problem
Currently, Typer optimizes single-command apps by executing them directly. In this specific execution path, the logic to invoke result_callback was missing, causing the callback to be silently ignored.

Solution
Modified get_command in typer/main.py to wrap the command callback. The wrapper executes the original command, captures the result, and then manually invokes the result_callback if one is defined.

Testing
I tested this locally with a script that returns a value from a single command:

Created a Typer app with one command returning a string.
Attached a result_callback to print the result.
Verified that the callback executes correctly after the command finishes.

[Copilot]

Pull request overview

Fixes Typer’s single-command “fast path” so result_callback is executed when an app has exactly one command and is invoked without specifying the command name (Issue #445).

Changes:

• Wrap the generated Click command callback for single-command apps to capture the command result.
• Invoke the app-level result_callback manually after the command finishes, mirroring Click group behavior.

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[dilanmelvin]

Maintainers, could you please add the bug label?
This PR fixes Issue #445 where result_callback was ignored for single-command apps.

[svlandeg]

@dilanmelvin: you've asked for a review by copilot, so can you address its review comments?

[dilanmelvin]

@svlandeg The review is done as per the request from copilot ,Thankyou