fix: make expect(..., message) consistent as error message prefix

[hi-ogawa]

Description

Minor behavior nit. Current behavior is:

expect(1, 'custom').toBe(2)
// AssertionError: custom: expected 1 to be 2 // Object.is equality

expect('bar', 'custom').toBeFoo()
// Error: custom

expect('value', 'custom').toMatchSnapshot()
// Error: Snapshot `...` mismatched

expect({ value: 1 }, 'custom').toMatchSnapshot({
  value: expect.any(String),
})
// Error: custom

This PR aligns custom matcher and built-in snapshot behavior to follow Chai style:

expect(1, 'custom').toBe(2)
// AssertionError: custom: expected 1 to be 2 // Object.is equality

expect('bar', 'custom').toBeFoo()
// Error: custom: <matcher-generated-message>

expect('value', 'custom').toMatchSnapshot()
// Error: custom: Snapshot `...` mismatched

expect({ value: 1 }, 'custom').toMatchSnapshot({
  value: expect.any(String),
})
// Error: custom: Snapshot properties mismatched

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[sheremet-va]

Is this a breaking change? The message in toThrowInlineSnapshot seems to change and I'm not sure if it's a valid pattern outside of Vitest's monorepo

[hi-ogawa]

I don't think this is a breaking change. I'd consider users catching and probing assertion error content is in a same territory as probing reporter output.