[python] Enum member names drop underscores before date segments

- `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.