fix: make view engine extension matching case-insensitive

[erdinccurebal]

Summary

• app.engine('Eta', fn) stored the engine under .Eta, but path.extname('search.eta') returns .eta, causing a lookup miss and triggering an unnecessary require() fallback.
• This normalizes extensions to lowercase in app.engine() and the View constructor so that engine registration and file extension matching are case-insensitive.

Before

app.engine('Eta', Eta.renderFile);
console.log(app.engines); // { '.Eta': [Function: renderFile] }

// When rendering 'search.eta':
// path.extname('search.eta') => '.eta'
// app.engines['.eta'] => undefined (BUG: stored as '.Eta')
// Falls through to require('eta').__express

After

app.engine('Eta', Eta.renderFile);
console.log(app.engines); // { '.eta': [Function: renderFile] }

// When rendering 'search.eta':
// path.extname('search.eta') => '.eta'
// app.engines['.eta'] => [Function: renderFile] (MATCH)

Changes

• lib/application.js: normalize extension to lowercase in app.engine()
• lib/view.js: normalize this.ext to lowercase in View constructor (both from filename and from defaultEngine)
• test/app.engine.js: 2 new tests for uppercase and mixed-case engine extensions

Test plan

• Added test: engine registered with uppercase ext matches lowercase file
• Added test: engine registered with mixed case ext works with view engine setting
• All 1246 tests pass (1244 existing + 2 new)

Fixes #4594