feat(providers): add optional AccountCredentialField for self-describing plugins

[hyoshi]

Summary

Provider plugins (built-in google_ads / meta_ads, and third-party plugins discovered via the mureo.providers entry-point group) can now declare an optional account_credential_fields: tuple[AccountCredentialField, ...] class attribute so introspection tooling — the mureo providers … CLI, configuration wizards, plugin authoring guides — can render setup prompts, validate config, and document plugins without hardcoding per-provider knowledge.

Plugins declare their per-account credential needs so tooling can render setup prompts, validate config, and document plugins without hardcoding per-provider knowledge. Existing providers without this declaration behave unchanged.

from mureo.core.providers import AccountCredentialField

class MyAdsProvider:
    name = "my_ads"
    display_name = "My Ads"
    capabilities = frozenset({...})
    account_credential_fields = (
        AccountCredentialField(
            key="advertiser_id",
            display_name="Advertiser ID",
            placeholder="adv-12345",
            required=True,
            description="From the MyAds dashboard.",
        ),
    )

Public surface added to mureo.core.providers

Name	Kind	Purpose
AccountCredentialField	frozen dataclass	One per-account credential descriptor: key, display_name, placeholder="", required=False, description="". Primitive str / bool only so dataclasses.asdict is JSON-serialisable.
get_account_credential_fields(provider)	function	Defensive accessor — returns () for providers that do not declare the attribute, raises TypeError when the declaration is malformed (non-tuple or wrong element type).

BaseProvider Protocol body is unchanged. The new attribute is documented as optional in the BaseProvider docstring and is read via getattr so @runtime_checkable isinstance() results do not flip for any existing plugin.

Built-in adapter updates

• mureo.adapters.google_ads.GoogleAdsAdapter declares customer_id (placeholder "123-456-7890", required). MCC login_customer_id is intentionally excluded — it identifies the operator's manager account and is shared across every Google Ads account the operator manages (operator-shared OAuth-level credential, not per-account).
• mureo.adapters.meta_ads.MetaAdsAdapter declares ad_account_id (placeholder "act_1234567890", required). Token / app id / app secret are operator-shared in the common Business Manager setup and live outside this declaration.

Backward compatibility

• Plugins that do not declare account_credential_fields continue to register and discover unchanged; get_account_credential_fields() returns (), which downstream tooling treats as "no per-account configuration needed."
• The BaseProvider Protocol body is unchanged so @runtime_checkable isinstance(obj, BaseProvider) keeps returning the same value for every existing plugin.
• get_account_credential_fields validation raises TypeError only when a plugin actively declares a malformed attribute — pure additive footprint for everyone else.

Plugin author docs

docs/plugin-authoring.md §3 (BaseProvider) gains a Declaring per-account credential fields (optional) subsection covering the dataclass shape, default values, and the accessor's defensive-read / strict-validation semantics — plus the operator-shared / per-account credential split shown above.

Test plan

• CI lint (ruff check mureo/, black --check mureo/) — clean locally
• CI tests (Python 3.10 / 3.11 / 3.12 + Windows) — 11 new unit tests + 2 import-allowlist updates; 394 tests pass across tests/core/providers/ + tests/adapters/
• CodeQL — no new findings expected; net additions are pure-Python @dataclass(frozen=True) + a defensive getattr-based accessor. No eval / subprocess / shell / network.
• Manual: drop into a Python shell, import from mureo.core.providers import get_account_credential_fields, AccountCredentialField, run get_account_credential_fields(GoogleAdsAdapter) and confirm it returns the declared tuple.
• Manual: install a pre-feature third-party plugin (anything that does not declare account_credential_fields); confirm discovery still loads it and get_account_credential_fields(...) returns ().