Python: Reject every multi-source combination in detect_media_type_from_base64

[sxh (sxh313)]

Motivation & Context

detect_media_type_from_base64() documents that exactly one of data_bytes, data_str, or data_uri
must be provided, and raises ValueError otherwise. That contract was not enforced for one
combination: data_str together with data_uri.

The guard compared data is not None, but data only ever mirrors data_bytes. When a data URI is
supplied, its payload is rebound into the data_str local variable (prefix, data_str = data_uri.split(",", 1)),
so by the time the data_str branch runs, the caller's data_str is gone. The call was accepted and the
URI silently won:

>>> detect_media_type_from_base64(
...     data_str="invalid_base64!",
...     data_uri="data:image/png;base64,iVBORw0KGgoAAAANSUhEUg==",
... )
'image/png'   # no error, the caller's base64 argument was discarded

This is the root cause of the second block reported in #5115: five of the six documented cases sat inside a
single with pytest.raises(...) block, so only the first ever executed and the bug stayed invisible.

Description & Review Guide

• What are the major changes?
  • python/packages/core/agent_framework/_types.py: the mutual-exclusion check inside the data_uri branch
    now also covers a caller-supplied data_str, i.e. it runs before the URI payload is rebound into that
    variable.
  • python/packages/core/tests/core/test_types.py: the two multi-statement pytest.raises blocks are split
    into one parametrized test that exercises every rejected combination (each in its own raises block) plus a
    regression test that asserts the specific data_str + data_uri case.
• What is the impact of these changes?
  Every input that supplies zero or more than one source now raises
  ValueError("Provide exactly one of data_bytes, data_str, or data_uri."), matching the docstring. Single-source
  behavior is unchanged. No new error type, message, or signature change, so callers that used the API as
  documented are unaffected.
• What do you want reviewers to focus on?
  Whether data_bytes + data_str + data_uri (all three) should keep the same message, which it does here,
  and whether you prefer this ordering fix over hoisting a single upfront sum(...) != 1 guard.

Deliberately out of scope: the first block of #5115 also notes that data_str="" returns None rather than
raising. That needs the maintainer decision recorded in the issue (whether an empty string is valid input or
should be rejected), so this PR only removes the dead statement and leaves the behavior as is.
The closed PR #5171 changed tests only; it did not fix the source and did not cover the silent
data_str + data_uri acceptance.

Related Issue

Fixes #5115

Contribution Checklist

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• All unit tests pass, and I have added new tests where possible
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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation enforces the documented contract and the tests cover every invalid source combination.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes mutual-exclusion validation for base64 media-type detection and strengthens regression coverage.

Changes:

• Rejects data_str combined with data_uri.
• Parametrizes all invalid source combinations.
• Removes previously unreachable test code.

File	Description
python/​packages/​core/​agent_framework/​_types.py	Validates conflicting inputs before rebinding URI data.
python/​packages/​core/​tests/​core/​test_types.py	Tests each invalid input combination independently.

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