Add gpt-5.3-codex model configuration

[Copilot]

GitHub Copilot now supports gpt-5.3-codex but the extension's configuration files were missing entries for token estimation and pricing.

Changes

• tokenEstimators.json: Added gpt-5.3-codex with 0.25 ratio (standard for GPT-5 family)
• modelPricing.json: Added gpt-5.3-codex with $1.75/$14.0 per million tokens (input/output), following gpt-5.2-codex pricing pattern

Configuration Details

{
  "gpt-5.3-codex": {
    "inputCostPerMillion": 1.75,
    "outputCostPerMillion": 14.0,
    "category": "GPT-5 models",
    "tier": "premium",
    "multiplier": 1
  }
}

Pricing aligned with gpt-5.2-codex as the most comparable model in the GPT-5 Codex series.

Original prompt

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This section details on the original issue you should resolve

<issue_title>🤖 Action Required: Update Copilot Supported Models</issue_title>
<issue_description>### 🚨 Missing Models in tokenEstimators.json
The following models are listed in the GitHub Copilot documentation but are missing from src/tokenEstimators.json:

• gpt-5.3-codex

💰 Missing Models in modelPricing.json

The following models are listed in the GitHub Copilot documentation but are missing from src/modelPricing.json:

• gpt-5.3-codex

Action Required:
Please update the JSON configuration files with the latest models.

Source: GitHub Copilot Supported Models</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 🤖 Action Required: Update Copilot Supported Models #236

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