feat: add DaoXE OpenAI-compatible provider

[seven7763]

Description: (required)

• Add DaoXE as a first-class OpenAI-compatible provider in the AI Gateway.
• Fixed base URL: https://daoxe.com/v1 (Chat Completions path).
• Auth: Authorization: Bearer <DAOXE_API_KEY>.
• Model IDs are account-scoped — no static public model/price catalog in this PR.
• DaoXE is a multi-model multi-protocol API gateway; this provider wires the OpenAI-compatible Chat Completions surface that Portkey routes through open-ai-base. The DaoXE platform also exposes Anthropic Messages and other protocols for clients that speak those APIs.
• Not available in mainland China.
• Pattern mirrors existing lightweight OAI providers (e.g. aibadgr): api.ts + chatComplete params/stream transform + VALID_PROVIDERS + providers.json metadata.

Usage (after merge):

from portkey_ai import Portkey

client = Portkey(
    provider="daoxe",
    Authorization="<DAOXE_API_KEY>",
)
# model = exact ID from the user's DaoXE account / GET https://daoxe.com/v1/models

Tests Run/Test cases added: (required)

• Static review against aibadgr provider pattern
• providers.json validates as JSON; single additive entry only
• Live gateway e2e against a real DaoXE key (not run in this PR environment — happy to re-run if maintainers want a workflow test recipe)

Type of Change:

• New feature (non-breaking change which adds functionality)

Links

• Product: https://daoxe.com
• Client notes: https://github.com/seven7763/DaoXE-AI/blob/main/CLIENT_SETUP.md
• Related Dify Marketplace plugin PR: Add DaoXE model provider plugin v0.0.1 langgenius/dify-plugins#2708