test: Improve test coverage for tecDIR_FULL error in xrpld

[neti-soft-com]

High Level Overview of Change

The main goal of this PR is to improve test coverage for the xrpld application.

With the recent ability to simulate the tecDIR_FULL error, we can now comprehensively cover all cases where this error is returned. Consequently, this PR modifies production code solely to remove the now-obsolete LCOV_EXCL_LINE comments.

Context of Change

This PR is enabled by the implementation of #5935.
Once completed, it will resolve issue #5969.

Type of Change

• test: - The changes only affect unit tests.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

Test Plan

Verification is done by running the automated unit test suite.
Code coverage metrics will reflect the newly covered tecDIR_FULL error branches.

WIP Status & Future Tasks

Note: This PR is currently a Draft.
Pending tasks before marking as "Ready to merge":

• Cover remaining edge cases.
• Resolve existing merge conflicts.
• Code and PR polishing.
• Ensure all commits are squashed logically, follow the 50/72 formatting rule, and are signed via GPG.

[github-actions]

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[github-actions]

This PR has conflicts, please resolve them in order for the PR to be reviewed.

[github-actions]

All conflicts have been resolved. Assigned reviewers can now start or resume their review.