Typed ProviderMetadata enum + drop the google-fc- magic-string coupling

[yuanhao]

Deferred follow-ups from #33 (items 4 and 5). Items 1–3 shipped in #55 / v0.9.0.

1. Replace Option<serde_json::Value> with a typed ProviderMetadata enum

Content::ToolCall.provider_metadata is an untyped bag with no schema, no namespacing, and no compile-time link between metadata shape and producing provider. Suggested shape (from #33):

#[derive(Clone, Debug, Serialize, Deserialize)]
#[serde(tag = "provider", rename_all = "snake_case")]
pub enum ProviderMetadata {
    Gemini { thought_signature: String },
    // Anthropic { cache_control: ... },
    // OpenAi { reasoning_id: String },
}

Implementation notes:

• Needs a serde back-compat path for persisted messages that carry the current raw-JSON shape (untagged fallback or a migration).
• Both google.rs paths change (capture in the stream loop, echo in content_to_google_parts / build_request_body).
• Since v0.9.0, Content::ToolCall is #[non_exhaustive] and constructed via Content::tool_call_with_metadata(), so this migration is no longer a hard downstream break — it can ship in any minor release.

2. Drop the google-fc- magic-string coupling

content_to_google_parts and build_request_body detect synthetic tool-call IDs via id.starts_with("google-fc-"). Fold a was_synthetic marker into the typed metadata (rides along naturally with item 1) instead of coupling formatter and serializer through a literal prefix.

When to do this

Trigger: the moment a second provider needs provider_metadata (e.g. OpenAI reasoning IDs) — do both items together before shipping that provider, since that is when the key-collision risk becomes real. Until then this is not urgent.