Removed `upb_EpsCopyInputStream_GetInputPtr()` from the public API.

In its place, we now provide higher-level operations with clearer semantics:

- `upb_EpsCopyInputStream_ReadStringAlwaysAlias()` -- for advancing past a region of known size and obtaining a `upb_StringView` pointing to the input buffer.
- `upb_EpsCopyInputStream_StartCapture()/EndCapture()` -- for marking a "start" position before advancing past some data of unknown size (eg. for skipping groups).  When the capture is ended, the user can obtain a `upb_StringView` for the entire captured region pointing to the input buffer.

There are several benefits from this change:

1. The higher-level operations let users express their intent more directly, saving them from managing low-level details (like the surprising `IsDone()` check in `decode.c`).
2. If/when we extend EpsCopyInputStream to support multi-buffer input, this will make it clearer what call sites need to change to handle multiple buffers correctly, and how to extend those higher-level operations to be multi-buffer-safe.
3. This will help us unwind some complexity around `IsDoneFallbackInline()`.  One of the original motivations for providing a custom buffer flip callback was to copy data out of the soon-to-disappear buffer if we were in the middle of capturing some unknown data.  If the higher level `StartCapture()/EndCapture()` operation can handle any required copying automatically, that puts us one step closer to having a single `IsDoneFallback()` with no need to let the user inject their own buffer flip callback.

This CL also fixes a bug in `_upb_WireReader_SkipGroup()`, and removes the code path where we parse unknown groups by calling back into `_upb_Decoder_DecodeMessage()` with a NULL MiniTable.

PiperOrigin-RevId: 833458789