fix(path_generator): prevent path truncation before current lanelet

[ToshifumiTeranishi]

Description

This PR fixes an issue in path_generator where the path is prematurely truncated before reaching the ego vehicle's current lanelet, causing a Failed to build trajectory from path points error.

Since the introduction of RouteManager in PR autowarefoundation#803, the lanelet sequence is provided and processed as a single continuous array ([backward] -> [current] -> [forward]). However, the forward path truncation logic calculates the accumulated distance s starting from the very beginning of this sequence. If the backward lanelets are physically long, s can exceed the truncation threshold (s_end) before the loop reaches the ego's current_lanelet. This triggers an early break and erases the ego's current and forward lanelets.

To resolve this, a current_lanelet_seen guard flag was added in the sequence iteration loop. This ensures that the forward path truncation (lanelets.erase and break) only triggers after the ego vehicle's current lanelet has been explicitly passed and the s_end threshold is correctly updated.

Related links

Internal: https://star4.slack.com/archives/C017VB9UG1L/p1773728712843039

How was this PR tested?

• After the fix:
  https://evaluation.tier4.jp/evaluation/reports/d62e78b0-3b31-5c6b-8f05-b560bc5e2d32?project_id=X8ft5pPN
• Before the fix:
  https://evaluation.tier4.jp/evaluation/reports/31af6aad-f4b4-5f21-9ece-70af0a66e416?project_id=X8ft5pPN

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

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