fix(search): encode cabin class in tfs booking deep-links

[Fanfulla]

Problem

build_tfs_token always writes protobuf field 9 as 1 (economy). Booking URLs built after a premium/business/first search therefore open the economy booking page.

Reproduced on current main by decoding a generated tfs token: the wire bytes contain 0x40 0x01 0x48 0x01 0x70 0x01 (f8=1, f9=1, f14=1) regardless of the cabin used for the search. The same hardcode exists in the JS port.

Fixes #213.

Cause

Field 9 is the seat-class slot (1 economy, 2 premium, 3 business, 4 first), matching SeatType. The encoder never took a cabin argument, and neither the CLI nor the MCP server passed the search's seat_type into build_flight_booking_url.

Change

• build_tfs_token(..., seat=1) writes field 9 from seat. Default remains economy so captured golden tokens stay byte-equal.
• SearchFlights.build_flight_booking_url(..., seat_type=SeatType.ECONOMY) forwards seat_type.value.
• CLI flights and MCP search_flights / get_booking_options pass the search cabin through.
• Same change in the JS port (buildTfsToken({ seat }), buildFlightBookingUrl({ seatType })).

Passenger count is still hardcoded (field 8). That is a nested message and is out of scope here, as noted in #213.

Tests

Executed locally:

• uv run pytest -vv tests/search/test_proto.py tests/search/test_booking_url.py tests/cli/test_flights.py — 74 passed
• uv run pytest -q tests/search/test_proto.py::TestBuildTfsToken::test_seat_business_encodes_field_9 tests/search/test_booking_url.py::TestBuildFlightBookingUrl::test_seat_type_business_changes_tfs_field_9 tests/cli/test_flights.py::test_flights_with_cabin_class — 3 passed
• Sabotage: hardcoding field 9 back to 1 made the two new encoder tests fail; restoring the fix made them pass
• uv run ruff check / ruff format --check on the touched Python files — clean
• bun test tests/search/proto.test.ts tests/search/flights_booking_url.test.ts — 55 passed
• bun run typecheck — clean

Golden tfs fixtures (economy captures) remain byte-perfect.

Greptile Summary

This PR propagates the selected cabin class into deterministic Google Flights booking deep links while preserving economy as the backward-compatible default.

• Adds cabin-class encoding to Python and TypeScript tfs token builders.
• Forwards the search cabin through the Python API, CLI, and MCP booking-link paths.
• Adds Python and TypeScript coverage for default economy and explicit business-class tokens.

Confidence Score: 5/5

The PR appears safe to merge, with cabin values consistently propagated through the affected booking-link paths.

The supported cabin enums map to the expected numeric values, all repository booking-link adapters forward the search cabin, defaults preserve existing economy tokens, and no changed-code failure remains.

Important Files Changed

Filename	Overview
fli/search/_proto.py	Adds an optional seat value to Python token construction and writes it to protobuf field 9 while retaining the economy default.
fli/search/flights.py	Extends the Python booking-link API with a typed cabin argument and forwards its enum value to the token builder.
fli/cli/commands/flights.py	Passes the cabin used for the CLI search into each per-flight booking link.
fli/mcp/server.py	Preserves cabin intent in booking links returned by flight-search and booking-option MCP flows.
fli-js/src/search/proto.ts	Adds matching cabin-field encoding to the TypeScript token builder with a backward-compatible economy default.
fli-js/src/search/flights.ts	Adds a typed booking-link cabin option and forwards it into the TypeScript token.
tests/search/test_proto.py	Verifies Python field-9 encoding for the default and business cabins.
tests/search/test_booking_url.py	Verifies the Python booking-link API changes its token for business class.
tests/cli/test_flights.py	Covers cabin propagation from the CLI search into booking-link generation.
fli-js/tests/search/proto.test.ts	Verifies TypeScript field-9 encoding and backward-compatible economy output.
fli-js/tests/search/flights_booking_url.test.ts	Verifies the TypeScript booking-link API forwards business class correctly.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[CLI, MCP, or SDK caller] --> B[Typed cabin selection]
  B --> C[Booking URL builder]
  C --> D[Pass SeatType numeric value]
  D --> E[tfs protobuf encoder]
  E --> F[Field 9: cabin class]
  F --> G[Google Flights booking deep link]

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Context used (4)

• Knowledge Base — Flight and date search workflows
• Knowledge Base — Command-line interface
• Knowledge Base — MCP server integration
• Knowledge Base — TypeScript search client