Use routes_pathname_prefix for internal Alive URL in Dash app

[Chiragasourabh]

When running a Dash app behind a proxy, the internal alive check (used during app startup) fails if the app is initialized with only requests_pathname_prefix, or if requests_pathname_prefix differs from routes_pathname_prefix.

This results in a misleading error:

OSError: Address 'http://127.0.0.1:9090' already in use.
Try passing a different port to run.

This is due to the alive check attempting to construct the URL using requests_pathname_prefix, which may not route correctly internally.

Solution

This PR updates the internal alive URL construction logic to use routes_pathname_prefix, which is the correct prefix for server-side routing and internal endpoint resolution.

This

• Prevents startup failures when requests_pathname_prefix != routes_pathname_prefix
• Ensures internal health check (alive check) functions correctly
• Unblocks deployment in more complex reverse-proxy setups

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • task 1
  • task 2
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

💃 Looks good, thank you.