fix(plugins): warn when required env vars are missing, handle str/dict manifest entries

[ygd58]

Problem

_load_plugin() emitted only a DEBUG registration count, so a plugin whose required env vars were absent loaded silently (issue #2768).

Fix

Normalize requires_env entries (str or dict, matching hermes_cli/plugins_cmd.py:307-314) before checking the process environment. Emit a WARNING when any required env var is not set. The empty-registration note is demoted to DEBUG to avoid false positives for provider-only plugins (e.g. image_gen/fal) whose registrations are not tracked in LoadedPlugin.

Addresses all three points from the maintainer review of #2768: requires_env normalization for both manifest forms, correct registration surface accounting (no false WARNING for providers), and tests ported to tests/hermes_cli/test_plugins.py.

Verification

4 new tests: str-form missing env, dict-form missing env, present env no warning, provider-only no false-positive. 4/4 pass alongside 100 existing plugin tests.

Fixes #2768

[teknium1]

Thanks for carrying forward the manifest normalization and regression coverage. The generic missing-env diagnostic is still needed on current main: hermes_cli/plugins.py:1792-1821 registers plugins and logs only DEBUG counts.

Problems

• The new missing-env check is after register_fn(ctx). A plugin that directly reads a missing variable can raise into the existing generic failure handler (hermes_cli/plugins.py:1824-1829) before the declared variable is reported. Move the normalization/warning before registration and cover a raising register() test.
• This does not fix the linked Hindsight path: general discovery skips plugins/memory/ (hermes_cli/plugins.py:1329-1340), Hindsight declares requires_env: [] (plugins/memory/hindsight/plugin.yaml:6), and plugins.memory owns its loader (plugins/memory/__init__.py:157-215). Please narrow the claim or add the diagnostic at that loader.
• The provider-only test's register() is a no-op, not a provider registration. Provider registrations are not represented in LoadedPlugin (hermes_cli/plugins.py:323-326; image providers register via :648-671).

Suggested changes

• Warn before calling register_fn(ctx) and add a throwing-registration regression test.
• Add a real provider-registration test, and separately decide whether the zero-registration diagnostic belongs in the memory-provider path.

Automated hermes-sweeper review.

[teknium1]

This validation runs after register_fn(ctx). Move missing-env normalization and its warning before registration: a plugin that accesses a missing required variable directly will raise into the generic load-error path before this branch can name the declared variable.

[teknium1]

This is a no-op registration rather than a provider registration, so it cannot verify the stated provider-only case. Exercise a real supported provider registration path, or name this test as a no-op plugin test.

[ygd58]

Ported forward in #68703: warning now fires before register_fn(ctx) so a plugin that crashes reading the missing var still reports the specific diagnostic, and the provider-only test now uses a real ImageGenProvider registration instead of a no-op. Narrowed the Hindsight claim -- that path uses a separate loader this PR does not touch. Closing in favor of #68703.