fix(model-normalize): pass DeepSeek V-series IDs through instead of folding to deepseek-chat

`_normalize_for_deepseek` was mapping every non-reasoner input into
`deepseek-chat` on the assumption that DeepSeek's API accepts only two
model IDs. That assumption no longer holds — `deepseek-v4-pro` and
`deepseek-v4-flash` are first-class IDs accepted by the direct API,
and on aggregators `deepseek-chat` routes explicitly to V3 (DeepInfra
backend returns `deepseek-chat-v3`). So a user picking V4 Pro through
the model picker was being silently downgraded to V3.

Verified 2026-04-24 against Nous portal's OpenAI-compat surface:
  - `deepseek/deepseek-v4-flash` → provider: DeepSeek,
    model: deepseek-v4-flash-20260423
  - `deepseek/deepseek-chat`     → provider: DeepInfra,
    model: deepseek/deepseek-chat-v3

Fix:
- Add `deepseek-v4-pro` and `deepseek-v4-flash` to
  `_DEEPSEEK_CANONICAL_MODELS` so exact matches pass through.
- Add `_DEEPSEEK_V_SERIES_RE` (`^deepseek-v\d+(...)?$`) so future
  V-series IDs (`deepseek-v5-*`, dated variants) keep passing through
  without another code change.
- Update docstring + module header to reflect the new rule.

Tests:
- New `TestDeepseekVSeriesPassThrough` — 8 parametrized cases covering
  bare, vendor-prefixed, case-variant, dated, and future V-series IDs
  plus end-to-end `normalize_model_for_provider(..., "deepseek")`.
- New `TestDeepseekCanonicalAndReasonerMapping` — regression coverage
  for canonical pass-through, reasoner-keyword folding, and
  fall-back-to-chat behaviour.
- 77/77 pass.

Reported on Discord (Ufonik, Don Piedro): `/model > Deepseek >
deepseek-v4-pro` surfaced
`Normalized 'deepseek-v4-pro' to 'deepseek-chat'`. Picker listing
showed the v4 names, so validation also rejected the post-normalize
`deepseek-chat` as "not in provider listing" — the contradiction
users saw. Normalizer now respects the picker's choice.

tests/hermes_cli/test_model_normalize.py

@@ -9,6 +9,7 @@ from hermes_cli.model_normalize import (
     normalize_model_for_provider,
     _DOT_TO_HYPHEN_PROVIDERS,
     _AGGREGATOR_PROVIDERS,
+    _normalize_for_deepseek,
     detect_vendor,
 )
 
@@ -191,3 +192,72 @@ class TestDetectVendor:
     ])
     def test_detects_known_vendors(self, model, expected):
         assert detect_vendor(model) == expected
+
+
+# ── DeepSeek V-series pass-through (bug: V4 models silently folded to V3) ──
+
+class TestDeepseekVSeriesPassThrough:
+    """DeepSeek's V-series IDs (``deepseek-v4-pro``, ``deepseek-v4-flash``,
+    and future ``deepseek-v<N>-*`` variants) are first-class model IDs
+    accepted directly by DeepSeek's Chat Completions API. Earlier code
+    folded every non-reasoner name into ``deepseek-chat``, which on
+    aggregators (Nous portal, OpenRouter via DeepInfra) routes to V3 —
+    silently downgrading users who picked V4.
+    """
+
+    @pytest.mark.parametrize("model", [
+        "deepseek-v4-pro",
+        "deepseek-v4-flash",
+        "deepseek/deepseek-v4-pro",          # vendor-prefixed
+        "deepseek/deepseek-v4-flash",
+        "DeepSeek-V4-Pro",                    # case-insensitive
+        "deepseek-v4-flash-20260423",         # dated variant
+        "deepseek-v5-pro",                    # future V-series
+        "deepseek-v10-ultra",                 # double-digit future
+    ])
+    def test_v_series_passes_through(self, model):
+        expected = model.split("/", 1)[-1].lower()
+        assert _normalize_for_deepseek(model) == expected
+
+    def test_deepseek_provider_preserves_v4_pro(self):
+        """End-to-end via normalize_model_for_provider — user selecting
+        V4 Pro must reach DeepSeek's API as V4 Pro, not V3 alias."""
+        result = normalize_model_for_provider("deepseek-v4-pro", "deepseek")
+        assert result == "deepseek-v4-pro"
+
+    def test_deepseek_provider_preserves_v4_flash(self):
+        result = normalize_model_for_provider("deepseek-v4-flash", "deepseek")
+        assert result == "deepseek-v4-flash"
+
+
+# ── DeepSeek regressions (existing behaviour still holds) ──────────────
+
+class TestDeepseekCanonicalAndReasonerMapping:
+    """Canonical pass-through and reasoner-keyword folding stay intact."""
+
+    @pytest.mark.parametrize("model,expected", [
+        ("deepseek-chat", "deepseek-chat"),
+        ("deepseek-reasoner", "deepseek-reasoner"),
+        ("DEEPSEEK-CHAT", "deepseek-chat"),
+    ])
+    def test_canonical_models_pass_through(self, model, expected):
+        assert _normalize_for_deepseek(model) == expected
+
+    @pytest.mark.parametrize("model", [
+        "deepseek-r1",
+        "deepseek-r1-0528",
+        "deepseek-think-v3",
+        "deepseek-reasoning-preview",
+        "deepseek-cot-experimental",
+    ])
+    def test_reasoner_keywords_map_to_reasoner(self, model):
+        assert _normalize_for_deepseek(model) == "deepseek-reasoner"
+
+    @pytest.mark.parametrize("model", [
+        "deepseek-chat-v3.1",    # 'chat' prefix, not V-series pattern
+        "unknown-model",
+        "something-random",
+        "gpt-5",                 # non-DeepSeek names still fall through
+    ])
+    def test_unknown_names_fall_back_to_chat(self, model):
+        assert _normalize_for_deepseek(model) == "deepseek-chat"