feature: Add optional PSR-3 message placeholder interpolation to TestLogger

[sasezaki]

Introduces an optional placeholder interpolation feature that allows TestLogger to interpolate context values into message placeholders.
(e.g., "User {username} created" with context ['username' => 'bolivar']).
This makes it easier to test log messages with dynamic values.

I often want asserting placeholder interpolation message.

$this->assertTrue($logger->hasDebugThatContains('User bolivar created'));

[Jean85]

I'm not sure this behavior is in-spec for PSR-3...

[Seldaek]

@Jean85 how so?

[Jean85]

You're right @Seldaek, I re-read the spec and it specifically allows this use case:

The message MAY contain placeholders which implementors MAY replace with values from the context array.