fix(tools): remove non-string enum from pushover priority for Gemini compat

The pushover tool priority parameter schema used integer enum values
[-2, -1, 0, 1, 2]. OpenAI-compatible APIs accept this, but the Gemini
API (and Gemini-relay proxies) strictly require all enum values to be
strings, rejecting the request with 400 Bad Request.

This causes every agent turn to fail with a non_retryable error when
using Gemini models, regardless of user message content, because tool
schemas are included in every request.

Fix: remove the enum constraint, keeping integer type and description
documenting the valid range. This is valid for both OpenAI and Gemini
providers and requires no changes to execute() which already uses
as_i64() with range validation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/tools/pushover.rs

@@ -99,7 +99,6 @@ impl Tool for PushoverTool {
                 },
                 "priority": {
                     "type": "integer",
-                    "enum": [-2, -1, 0, 1, 2],
                     "description": "Message priority: -2 (lowest/silent), -1 (low/no sound), 0 (normal), 1 (high), 2 (emergency/repeating)"
                 },
                 "sound": {