fix(invoke5): warn on unknown v5 fields without breaking parsing

[lstein]

Why this exists

`GenerationMetadata5` carries `extra="allow"` intentionally — InvokeAI's v5 metadata schema is actively extended between releases, and silently dropping a new upstream field is preferable to refusing to parse the image. The trade-off has been zero visibility into schema drift: new fields slipped through entirely unnoticed.

What lands

A `model_validator(mode="after")` hook on `GenerationMetadata5` walks `self.pydantic_extra` (Pydantic v2's storage for `extra="allow"`'d fields) and logs each unknown field name once per process via a module-level `_warned_extra_fields` set. Subsequent images carrying the same field are silent — the goal is "did anything new show up?", not per-image instrumentation, so log volume stays proportional to the number of distinct schema additions seen.

Sample log output (single warning per field, lifetime of the process):

```
WARNING invoke5metadata: InvokeAI v5 metadata contains field(s) not declared
in GenerationMetadata5: new_field_x, new_field_y. Parsing succeeded
via extra='allow'; add to the schema if useful in the drawer or
recall payload.
```

Behavior preserved

• Parsing still succeeds for any v5 payload regardless of unknown fields.
• The discriminated union, the formatter, and `to_recall_payload` are untouched.
• `extra="allow"` stays — this PR is observability, not enforcement.

Test plan

• `ruff check` — clean
• `pytest tests/backend/test_invoke_metadata.py` — 48 passed (4 new)
• `pytest tests/backend` — 260 passed (was 256; the 4 added tests are the only diff)
• Manual: re-index an album of recent InvokeAI v5 images and watch the log for any `InvokeAI v5 metadata contains field(s)` warnings — likely surfaces fields worth adding to the schema if any have been added upstream since the last update.

Four new tests cover the contract:

Scenario	Asserted
Unknown fields in a payload	warning fires, message lists them
Only known fields	no warning
Same field seen twice	second parse is silent
New field after a known one	warning fires for the new one only

The fixture clears `_warned_extra_fields` around each test so test order doesn't influence what counts as first-seen.

Net +119 lines (38 in the schema file for the validator + module-level comment, 81 for the four tests).

🤖 Generated with Claude Code