Add keepTrailingNewline option to LegacyOverrides to match Python Jinja2 default behaviour

[jmoraleda]

Description:

By default, Python Jinja2 strips a single trailing newline from rendered output (keep_trailing_newline=False). Jinjava has historically preserved trailing newlines, causing a subtle but consistent difference when porting templates from Python Jinja2.

This PR adds a keepTrailingNewline flag to LegacyOverrides that allows users to opt into Python-compatible behaviour.

Changes:

• LegacyOverrides: adds isKeepTrailingNewline(), defaulting to true (preserve existing Jinjava behaviour — no impact on current users).
• JinjavaInterpreter: applies the strip in stripTrailingNewlineIfNeeded(), called at the normal return points of the private render() method. Early-exit error paths (OutputTooBigException, CollectionTooBigException) are intentionally unaffected.
• TrailingNewlineTest: covers both flag values, the NONE/THREE_POINT_0/ALL presets, and edge cases (no trailing newline, multiple trailing newlines, rendered expressions).

Usage:

To opt into Python-compatible behaviour:

JinjavaConfig config = JinjavaConfig.newBuilder()
    .withLegacyOverrides(
        LegacyOverrides.newBuilder()
            .withKeepTrailingNewline(false)
            .build()
    )
    .build();

Compatibility note:

keepTrailingNewline is set to true (legacy behaviour) in THREE_POINT_0 to avoid breaking existing tests. It is set to false in ALL. Maintainers may wish to move it to false in THREE_POINT_0 as well if they consider stripping the trailing newline to be part of the canonical Python-compatible behaviour set — the change is intentionally conservative here to leave that decision to the maintainers.