Fix MaxTripLegs constraint allowing infinite same-mode expansion

[Copilot]

Multimodal search was being killed by the system due to unbounded memory growth during frontier expansion.

Root Cause

The MaxTripLegs constraint in constraint.rs incorrectly validated same-mode edge continuations:

let n_legs = match active_mode {
    Some(active_mode) if active_mode != edge_mode => n_existing_legs + 1,
    _ => 0,  // Bug: same-mode continuation always returns 0 legs
};
let is_valid = n_legs <= *max_legs;

When active_mode == edge_mode (continuing with the same travel mode), the code returned n_legs = 0, making the constraint check always pass regardless of actual trip leg count. This allowed infinite expansion within a single mode once any mode transition exceeded the limit.

Changes

• constraint.rs line 92: Changed _ => 0 to _ => n_existing_legs to properly validate same-mode continuations
• model.rs: Added test_max_trip_legs_same_mode_continuation_at_limit to verify same-mode edges are rejected when over the limit

The constraint now correctly bounds search expansion for both mode transitions and same-mode continuations.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>bugs during test of template TOML file</issue_title>
<issue_description>while attempting a test run from a bambam-omf import using a new template configuration TOML file as part of #96, we can get as far as running search. however, the app is killed by the system during search, without returning a single successful search result:

queries assigned per executor: [230, 230, 230, 230, 230, 230, 230, 229]
zsh: killed     RUST_LOG=info ../rust/target/release/bambam -c denver/bambam-config-omf.toml 

this is likely a search running without bounds. some possible issues:

• thrashing in the frontier space due to repeated labels
  • label model domination implementation needs to be finished
• solution size is not sufficient for a termination condition with label setting
• we should be able to run bambam with query-time assignment of available_modes, but, it seems it is only set at config time (see rust/bambam/src/model/frontier/multimodal/service.rs for the build() method implementation)</issue_description>

<agent_instructions>task: identify what could be causing an infinite loop or overflow of resources</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes bugs during test of template TOML file #97

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