feat: add GitHub pricing tools (github_pricing + github_cost_estimate)

[msftnadavbh]

Summary

Adds two new MCP tools for GitHub product pricing, bringing the total tool count from 13 to 15.

New Tools

Tool	Description
github_pricing	Full catalog lookup for Plans, Copilot, Actions runners, Advanced Security (GHAS), Codespaces, Git LFS, and Packages
github_cost_estimate	Monthly/annual cost estimation based on team size and usage (seats, Copilot licenses, Actions minutes, Codespaces hours, LFS packs, GHAS committers)

Key Design Decisions

• Static pricing table — GitHub pricing is not available via the Azure Retail Prices API, so prices are maintained as verified constants in config.py with a GITHUB_PRICING_DATA_VERSION stamp (2026-03-03)
• No auth required — all lookups are offline
• Natural-language aliases — e.g., ci/cd → Actions, pair programmer → Copilot, ghas → Security
• Follows Databricks package pattern — config → service → handlers mixin → formatters → tool definitions

Files Changed (14)

New (6):

• src/azure_pricing_mcp/services/github_pricing.py — GitHubPricingService with get_pricing() + estimate_cost()
• src/azure_pricing_mcp/github_pricing/__init__.py — package exports
• src/azure_pricing_mcp/github_pricing/tools.py — tool definitions
• src/azure_pricing_mcp/github_pricing/handlers.py — GitHubPricingHandlers mixin
• src/azure_pricing_mcp/github_pricing/formatters.py — Markdown formatters
• tests/test_github_pricing.py — 47 tests

Modified (8):

• src/azure_pricing_mcp/config.py — static pricing tables (Plans, Copilot, Actions, Security, Add-ons, aliases)
• src/azure_pricing_mcp/services/__init__.py — export GitHubPricingService
• src/azure_pricing_mcp/tools.py — append GitHub tool definitions
• src/azure_pricing_mcp/handlers.py — add GitHubPricingHandlers to mixin chain + dispatch
• src/azure_pricing_mcp/server.py — dispatch routes
• docs/TOOLS.md — new section, tool count 13 → 15
• docs/FEATURES.md — new GitHub Pricing Tools feature section
• CHANGELOG.md — v3.5.0 entry

Testing

All 235 tests pass (188 existing + 47 new):

• Config validation (table structure, required keys)
• Alias resolution (known aliases, unknown input, edge cases)
• Service logic (full catalog, filtered queries, cost calculations including free-tier deduction)
• Formatter output (Markdown structure, section filtering)
• Handler integration (end-to-end through mixin)