fix(gateway): suppress "(empty)" visible replies on chat surfaces

[livingghost]

What does this PR do?

This prevents Hermes from sending the literal string "(empty)" to messaging platforms when a reasoning-only completion produces no visible user-facing text.

Core Hermes intentionally stores reasoning-only completions with content = "(empty)" so providers and session history keep a non-empty assistant message. That behavior is useful internally, but it leaks a confusing placeholder into Discord and other chat surfaces.

This patch keeps the core/session-history behavior unchanged and only suppresses "(empty)" at the messaging boundary, making it behave like "no reply" for platform delivery.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Tests (adding or improving test coverage)

Changes Made

• Added _is_silent_response(...) in gateway/platforms/base.py
• Treat "(empty)" the same as None/empty at the platform delivery boundary
• Kept core run_agent.py behavior unchanged, preserving the semantics introduced by fix: accept reasoning-only responses without retries — set content to "(empty)" #5278
• Added regression tests in tests/gateway/test_platform_base.py for:
  • exact "(empty)"
  • whitespace-wrapped "(empty)"
  • normal visible responses still sending normally

How to Test

1. Run:
   python -m pytest -o addopts= tests/gateway/test_platform_base.py -q
2. Trigger a reasoning-only completion that would otherwise surface "(empty)"
3. Confirm no visible chat message is sent
4. Confirm regular non-empty replies still send normally

Tested on

• Windows 11 host
• Linux Docker container
• Manual Discord verification that reasoning-only completions no longer surface "(empty)"