test: add comprehensive unit tests for Template::Plugins

[toddr-bot]

What

49 unit tests for Template::Plugins — the plugin provider that loads and instantiates TT plugin modules.

Why

Template::Plugins had no unit tests. It's a critical load path: every [% USE Plugin %] directive routes through fetch() and _load(). The coverage gap meant regressions in factory caching, TOLERANT mode, and LOAD_PERL semantics were invisible.

How

Tests cover the full surface area without requiring disk-based .pm files:

• Constructor: TOLERANT, LOAD_PERL flags, PLUGIN_BASE scalar/array coercion, PLUGINS merge with STD_PLUGINS, PLUGIN_FACTORY pre-population
• fetch(): successful load, undef args default, FACTORY cache hit (single _load() call), CODE factory branch, prototype factory (object method new()), new() returning undef, new() dying
• TOLERANT mode: all error paths converted to STATUS_DECLINED
• PLUGIN_BASE search: multi-base resolution, dot-to-:: name conversion, case-insensitive lookup
• LOAD_PERL: plain module loaded, context stripped from constructor args, unloadable module error
• _load() errors: broken module name, TOLERANT vs non-tolerant handling
• Instance isolation: FACTORY cache not shared between separate Plugins objects

LOAD_PERL tests use %INC pre-registration to avoid disk-based require, keeping tests self-contained.

Testing

perl -Ilib t/unit-plugins.t — 49 tests pass. Full suite: 3435 tests, all green.

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Quality Report

Changes: 1 file changed, 407 insertions(+)

Code scan: clean

Tests: passed (OK)

Branch hygiene: clean

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