Add support for wildcard route handling - #8588

Description

This PR adds first-class dynamic route support across both server implementations and makes route context available inside apps in a backend-consistent way.

Users can now serve apps on dynamic routes and reliably read:

• pn.state.route_params for captured path variables
• pn.state.app_url as the concrete matched app path for the current session (not new)

It also improves route matching behavior for wildcard routes, ensures websocket/session flows preserve route context, and updates docs to present path-template syntax as the primary recommended style.

Supported route syntax

• Path-template syntax (recommended, cross-backend):

  • /user/{name}
  • /files/{filepath:path}

• Regex syntax (Tornado):

  • /user/([^/]+)
  • /org/(?P<org>[^/]+)/user/(?P<user>[^/]+)

For Tornado, path templates are normalized to Tornado-compatible patterns.
For FastAPI, path templates are passed through naturally and now preserve route metadata in session state.

Behavior improvements included

• Wildcard/dynamic routes no longer interfere with core app endpoints (/ws, /metadata, /autoload.js).
• Root index no longer auto-redirects to an invalid wildcard-only route.
• Session creation and websocket-first flows both carry route metadata so app state is consistent regardless of connection path.
• Server docs now include a dedicated wildcard routes how-to and backend compatibility guidance.

Examples

Tornado

import panel as pn
pn.extension()

def app():
    return {
        "route_params": pn.state.route_params,
        "app_url": pn.state.app_url,
    }

pn.serve({"/user/{name}": app}, show=False)
# /user/alice -> {"route_params": {"name": "alice"}, "app_url": "/user/alice"}

FastAPI

import panel as pn
from panel.io.fastapi import serve
pn.extension()

def app():
    return {
        "route_params": pn.state.route_params,
        "app_url": pn.state.app_url,
    }

serve({"/files/{filepath:path}": app}, show=False)
# /files/a/b/c.txt -> {"route_params": {"filepath": "a/b/c.txt"}, "app_url": "/files/a/b/c.txt"}

AI Disclosure

• Tool & Model: Cursor + Codex 5.3 / GPT 5.5

• Usage: Used to investigate routing internals, implement dynamic route support across Tornado/FastAPI, add tests, and update documentation.

• I have tested all AI-generated content in my PR.

• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing
• Added documentation