feat(mcp): add list_fees and get_fee tools

[vladmarascu]

Summary

Adds two read-only MCP tools that expose Lago's /fees endpoint to AI agents:

• list_fees — list with rich filters (fee type, billable metric code, customer, subscription, currency, payment status, created-at date range, pagination)
• get_fee — single fee by Lago ID (UUID)

This is the third PR in a chain. Depends on:

• getlago/lago-rust-client#44 — lago-types fee modules (merged, published as lago-types 0.1.23 via refactor(billable_metric): Use lago-client for update_billable_metric #45)
• getlago/lago-rust-client#47 — lago-client list_fees / get_fee methods (merged, published as lago-client 0.1.25)

Code changes

File	Change
mcp/src/tools/fee.rs (new)	ListFeesArgs, GetFeeArgs, FeeService with build_request, list_fees, get_fee handlers. Mirrors the existing invoice.rs pattern.
mcp/src/tools.rs	pub mod fee;
mcp/src/server.rs	Imports FeeService, adds field to LagoMcpServer, instantiates in new(), registers two #[tool(description=...)] methods.
mcp/Cargo.toml	Bumps lago-client to 0.1.25 and lago-types to 0.1.23.
README.md	New ### Fees section under Available Tools.

Tool descriptions (what the LLM reads)

The #[tool(description=...)] strings shape when the model decides to call these tools:

• list_fees description explicitly mentions "MRR calculations" and "selected usage charges (e.g., seats, storage)". This is the surface that connects "What's our MRR including seats?" to a list_fees call with billable_metric_code=seats.
• get_fee is a standard drill-down by Lago ID — used as a follow-up to a list_fees result.

Happy to adjust wording if there's a preferred phrasing.

What this PR does NOT do

• Mistral agent config sync. The function schemas in Mistral's agent console are hand-maintained today. After this merges and the MCP server is deployed, someone needs to paste the list_fees and get_fee schemas into the Mistral agent. Tracked as operational followup.
• Schema-level per_page max enforcement. The arg description says "max: 100" but the JSON Schema doesn't enforce it. Matches the existing pattern in invoice.rs and every other paginated tool. Worth a separate cleanup PR.
• Destructive-op safety scaffolding. Both new tools are read-only, so unaffected. The pattern still needs designing for the existing mutating tools — separate effort.

Test plan

Verified end-to-end locally against a real Lago account (4,521 fees in test data):

• cargo build — clean
• cargo fmt --all -- --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test — passes
• MCP server boots on port 3000, /health responds
• tools/list advertises both new tools (55 total, up from 53)
• tools/call list_fees with per_page=3 returned 3 real fees with valid pagination metadata
• tools/call get_fee with a UUID from the list response returned the matching single fee
• tools/call list_fees with fee_type=subscription correctly filtered server-side

Commits

0a3ee19 chore(mcp): bump lago-client to 0.1.25 and lago-types to 0.1.23
7621e71 feat(mcp): add list_fees and get_fee MCP tools