Surface low PRO API credit balance for read_contract

[akolotov]

Description

Sub-issue of #382 (part 3 of 3). Extend credit awareness (low-credits advisory note + 402 credit-exhaustion handling) to read_contract, which uses a separate transport and is intentionally excluded from parts 1 and 2.

Depends on part 1 (#393) and part 2 (#394), which establish CreditsExhaustedError, the CreditSink/ContextVar mechanism, and the low-credits note on the HTTP transport.

Background

read_contract does not go through _make_blockscout_http_request. It calls smart-contract functions via web3.py over the PRO API RPC endpoint through WEB3_POOL (blockscout_mcp_server/web3_pool.py). As a result:

• The x-credits-remaining header from the RPC responses is not captured, so read_contract responses never carry the low-credits note.
• A credit-related 402 on the RPC path would surface as a generic web3 exception, not CreditsExhaustedError.

This means the "applies to every tool" intent of #382 is consciously narrowed in parts 1–2; this issue closes that gap for the web3 transport.

Open questions to resolve

• Does the PRO API RPC path consume credits and return x-credits-remaining? (Note: /api/eth-rpc was rejected in direct_api_call per Reject on /api/eth-rpc in direct_api_call #389 — clarify how RPC requests are routed/priced for the web3 pool.)
• How to intercept web3 HTTP response headers (e.g. a custom AsyncHTTPProvider / request middleware on the pooled provider) so the value can be fed into the same CreditSink.
• How to detect and map an RPC-level credit rejection to CreditsExhaustedError.

Proposed Changes (to be refined after the open questions)

• Hook the WEB3_POOL provider so successful RPC responses feed x-credits-remaining into the per-invocation CreditSink (reusing the part-2 mechanism), making the low-credits note appear on read_contract responses.
• Map RPC credit-exhaustion to CreditsExhaustedError so read_contract produces the same distinct error as the HTTP-transport tools.

Tests

• read_contract populates the CreditSink from RPC responses and surfaces the low-credits note via build_tool_response.
• RPC credit-exhaustion maps to CreditsExhaustedError (and REST → 402).

Documentation

• SPEC.md: remove/replace the part-1/2 scope note that excludes the web3 path; document the unified behavior.