[python] Enum member names drop underscores before date segments

[iscai-msft]

• WEB_SEARCH_PREVIEW_2025_03_11 is emitted as WEB_SEARCH_PREVIEW2025_03_11 (underscore before the date dropped)
• DEFAULT_2024_11_15 is emitted as DEFAULT2024_11_15 (same issue)

Similarly A2A enum member gets mangled to A2_A
In AgentEndpointProtocol, the value A2A is being emitted as A2_A. The emitter is incorrectly inserting an underscore, probably from a regex that tries to split on transitions between letters and digits.

Part of issues @dargilco is dealing with in his post processing script.