Address review comment in Add CI test to validate Azure MCP tool names

Issue to address comment in Add CI test to validate Azure MCP tool names in skill markdown files

Address review comment - https://github.com/microsoft/GitHub-Copilot-for-Azure/pull/1709#discussion_r3101849540

>Greedy match is fine today because every azure__ in current skills is bounded by backticks or whitespace - I grepped. But if a skill ever writes the MCP function-call form azure__storage_blob_list as a single token, this will fail: greedy capture grabs storage_blob_list, which isn't in the snapshot (only storage is).
Two low-cost options if you want to harden it:
After extracting the suffix, try progressively shorter prefixes down to the first underscore before declaring invalid.
Leave it strict and document the convention in the snapshot header so authors know compound forms aren't supported.