Fix create_scenario reporting false failures and storing duplicates

[92Infinitus92]

The MCP tools ran HTTP through reqwest's blocking client from async tool functions.
A create_scenario the server had already stored (POST /v1/scenarios 200 in the
CLI log) could still be reported to the model as a connection failure — it retried,
and a second scenario landed under the same id, so the studio showed two identical
cards. All three affected tools now use the async client.

POST /v1/scenarios is idempotent by content: same id and same document is a
no-op, different content under a taken id returns 409. The check shares the write
lock with the insert, and create_scenario turns a 409 into an actionable message.

The template-registry read guard moved into its own scope — holding it across the
new await made the future non-Send.

Testing. cargo test: cli 20/20 (new test: create → identical retry →
conflicting content), mcp 5/5, core pyth 3/3; fmt clean. Live with Claude and
OpenAI: three scenarios in a row, one POST each, no retries, clean logs.

Not addressed here: MCP failures still come back as successful tool results
carrying an error payload — fixing that means honouring isError in the shared
client path used by all five providers, a separate change.

Greptile Summary

The PR makes scenario creation idempotent and migrates MCP HTTP operations from blocking to asynchronous reqwest calls.

• Returns a no-op success for identical scenario retries and a conflict for reused IDs with different content.
• Keeps the scenario write lock across duplicate detection and insertion.
• Releases the template-registry read guard before awaiting HTTP work.
• Converts token-account and scenario HTTP operations to async calls.
• Adds coverage for initial creation, identical retry, and conflicting content.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
crates/cli/src/http/mod.rs	Adds atomic duplicate detection and content-based idempotency to scenario creation, with focused endpoint coverage.
crates/mcp/src/surfpool/mod.rs	Migrates MCP HTTP calls to async reqwest, scopes template-registry locking before awaits, and maps scenario conflicts to an actionable response.
crates/mcp/src/surfpool/set_token_account.rs	Converts token-account RPC execution and response-body handling to asynchronous reqwest operations.

Sequence Diagram

sequenceDiagram
    participant MCP as MCP create_scenario
    participant API as POST /v1/scenarios
    participant Store as Scenario store
    MCP->>API: POST scenario
    API->>Store: Acquire write lock and find ID
    alt ID absent
        API->>Store: Insert scenario
        API-->>MCP: "200 {id}"
    else Same ID and document
        API-->>MCP: "200 {id} (no-op)"
    else Same ID, different document
        API-->>MCP: 409 conflict
        MCP-->>MCP: Return actionable error payload
    end

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