feat(providers): add BytePlus ModelArk (international) + coding endpoint (#78)

Adds two providers for BytePlus ModelArk, the international edition of
Volcano Engine, distinct from the existing cn-only `volcengine` provider:

- `byteplus`: standard `/api/v3` endpoint, 21 models total
  - Text (10): Seed 2.0 Pro/Mini/Lite/Code, Seed 1.8, Seed Translation,
    GLM-4.7, DeepSeek V3.2/V3.1, GPT-OSS-120B
  - Image (4): Seedream 3.0/4.0/4.5/5.0-lite (per-piece pricing in comments)
  - Video (7): Seedance 1.0/1.5 family + Dreamina Seedance 2.0/fast
- `byteplus_coding`: `/api/coding` Anthropic-compatible endpoint with
  9 friendly aliases (ark-code-latest auto-router, bytedance-seed-code,
  dola-seed-2.0-{pro,code,lite}, kimi-k2.5, glm-4.7, glm-5.1, gpt-oss-120b).

Both use BYTEPLUS_API_KEY env var. All listed model IDs were verified
to return HTTP 200 against the live ap-southeast endpoint. Pricing is the
standard real-time tier from the BytePlus console (a discounted batch tier
exists at roughly half the rate, not modeled here).

Excluded for follow-up (schema doesn't currently support these modalities):
- Skylark embedding-vision (no `embedding` modality in schema)
- Hyper3D-Gen2, Hitem3D-2.0 (no `3d` modality in schema)

providers/byteplus-coding.toml

@@ -0,0 +1,123 @@
+# BytePlus ModelArk — Coding Plan (Anthropic-compatible) — https://www.byteplus.com/en/product/ModelArk
+# International edition's Claude Code-compatible endpoint with friendly model aliases.
+# Models: 9 (auto-routed alias + 8 named models)
+#
+# Pricing matches the underlying versioned snapshots in byteplus.toml. The
+# Coding Plan endpoint exposes friendly stable names that route to the
+# current production snapshot, so versions advance silently without config
+# changes (use `ark-code-latest` to always get the best current routing).
+
+[provider]
+id = "byteplus_coding"
+display_name = "BytePlus ModelArk Coding Plan"
+api_key_env = "BYTEPLUS_API_KEY"
+base_url = "https://ark.ap-southeast.bytepluses.com/api/coding"
+key_required = true
+
+[[models]]
+id = "ark-code-latest"
+display_name = "Ark Code (auto-routed)"
+tier = "frontier"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 1.00
+output_cost_per_m = 6.00
+supports_tools = true
+supports_streaming = true
+aliases = ["ark-coding"]
+
+[[models]]
+id = "bytedance-seed-code"
+display_name = "ByteDance Seed Code"
+tier = "frontier"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 1.00
+output_cost_per_m = 6.00
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "dola-seed-2.0-pro"
+display_name = "Dola Seed 2.0 Pro"
+tier = "frontier"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 1.00
+output_cost_per_m = 6.00
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "dola-seed-2.0-code"
+display_name = "Dola Seed 2.0 Code"
+tier = "smart"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 1.00
+output_cost_per_m = 6.00
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "dola-seed-2.0-lite"
+display_name = "Dola Seed 2.0 Lite"
+tier = "smart"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 0.50
+output_cost_per_m = 4.00
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "kimi-k2.5"
+display_name = "Kimi K2.5"
+tier = "frontier"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 0.60
+output_cost_per_m = 2.50
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "glm-4.7"
+display_name = "GLM-4.7"
+tier = "smart"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 0.60
+output_cost_per_m = 2.20
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "glm-5.1"
+display_name = "GLM-5.1"
+tier = "frontier"
+context_window = 196608
+max_output_tokens = 8192
+input_cost_per_m = 0.80
+output_cost_per_m = 3.00
+supports_tools = true
+supports_streaming = true
+aliases = []
+
+[[models]]
+id = "gpt-oss-120b"
+display_name = "GPT-OSS 120B"
+tier = "smart"
+context_window = 131072
+max_output_tokens = 8192
+input_cost_per_m = 0.10
+output_cost_per_m = 0.50
+supports_tools = true
+supports_streaming = true
+aliases = []