feat(extensions): improve auth UX and add load-time validation (#536)

* feat(extensions): add load-time validation for auth capabilities

Catch common misconfigurations (missing auth section, missing setup_url,
short prompts) at startup via tracing::warn instead of silently failing
at auth time.

* feat(extensions): improve auth prompts, setup_url, and showAuthCard

Add setup_url and descriptive prompts to channel and tool capabilities
files. Fix showAuthCard in web gateway and improve extension manager
auth flow messaging.

* refactor(extensions): extract MIN_PROMPT_LENGTH constant in validate()

Address review feedback: replace magic number 30 with a named constant
for readability and maintainability.

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

channels-src/discord/discord.capabilities.json

@@ -6,15 +6,16 @@
     "required_secrets": [
       {
         "name": "discord_bot_token",
-        "prompt": "Enter your Discord Bot Token (from Developer Portal)",
+        "prompt": "Enter your Discord Bot Token. Find it under Bot > Token in your Discord Application settings.",
         "optional": false
       },
       {
         "name": "discord_public_key",
-        "prompt": "Enter your Discord Application Public Key (from Developer Portal > General Information)",
+        "prompt": "Enter your Discord Application Public Key (found under General Information in your Discord Application settings).",
         "optional": false
       }
-    ]
+    ],
+    "setup_url": "https://discord.com/developers/applications"
   },
   "capabilities": {
     "http": {