feat(router)!: add UseOuter middleware layer and update Predicate signature

[fluffur]

This pull request introduces a new global middleware layer called UseOuter and updates the Predicate signature to be context-aware.

---

Key Changes

1. Added UseOuter Middleware: Introduced bot.UseOuter(), which allows registering global middlewares that run before route matching. This is useful for firewalls, blacklists, or pre-loading database configurations.
2. Context in Predicates: Changed the Predicate signature so it now accepts *botapi.Context instead of *botapi.Update. This allows route guards to read data injected into the context by the new UseOuter middlewares (for example, custom command prefixes).

Execution Order

An incoming update now passes through the bot in this sequence:
bot.UseOuter() -> bot.Predicate() matching -> bot.Use() -> bot.Handler

---

BREAKING CHANGES and Migration Guide

The Predicate type signature has changed from func(u *Update) bool to func(c *Context) bool. Existing custom predicates will cause compilation errors and must be updated.

How to migrate:

Change the function argument from *botapi.Update to *botapi.Context. If your code requires raw update fields, access them through c.Update or use context helper methods.

// Old syntax
func MyPredicate(u *botapi.Update) bool {
    return u.Message != nil
}

// New syntax
func MyPredicate(c *botapi.Context) bool {
    return c.Update.Message != nil
}

---

Documentation and Examples

• Added an "Outer Middleware" section to README.md explaining the new pipeline lifecycle.
• Created a standalone example in examples/middleware/main.go to demonstrate how data moves from UseOuter to predicates and handlers.

[codecov]

Codecov Report

❌ Patch coverage is 98.18182% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.81%. Comparing base (3466ee3) to head (2e6a290).

Files with missing lines	Patch %	Lines
on.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
+ Coverage   78.78%   78.81%   +0.02%     
==========================================
  Files         118      118              
  Lines        8316     8325       +9     
==========================================
+ Hits         6552     6561       +9     
  Misses       1237     1237              
  Partials      527      527              

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