feat(trajectory_validator): combine validator and concatenator

[zulfaqar-azmi-t4]

Description

As part of the effort to reduce the number of running nodes, this PR merges the trajectory concatenator functionality into the Trajectory Validator.

The concatenator is refactored into a pure C++ library (autoware::trajectory_concatenator::TrajectoryConcatenator) — independent of rclcpp — and is consumed by the Validator as the first stage of its multi-stage pipeline. The standalone concatenator node, its launch file, and its config yaml are removed; the concatenator parameters are now declared by the Validator node.

Related links

None

How was this PR tested?

Run PSIM with diffusion planner

Notes for reviewers

None.

Interface changes

• removed trajectory_concatenatory.launch.xml and trajectory_concatenator.param.yaml

Effects on system behavior

• One fewer process/node in the planning pipeline. The concatenation stage now runs in-process inside the Trajectory Validator, eliminating one inter-process hop and the associated serialization/transport overhead.
• The concatenator stage's output cadence is unchanged: it is still flushed by a fixed-rate timer (the same ~30ms cadence noted in the new trajectory_concatenator.hpp), so the average / max latency floor of the stage itself is the same.
• No semantic behavior change is expected: the concatenation logic (most-recent-per-generator buffer with time-based stale pruning) is identical to the previous standalone node; only its hosting process has changed.

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