refactor(core): remove whitespace trimming from path normalization and rename

[erickguan]

Which issue does this PR close?

Implements RFC 7799's path normalization part.

Also closes #6577

Rationale for this change

• Improve readability and documentation
• More defined path normalization conventions, rules, and who should use what.

What changes are included in this PR?

• Remove .trim() from normalize_path to preserve whitespace in path components, aligning with POSIX, URI, and object-store conventions where whitespace is significant content.
• Add . segment filtering to both normalize_path and normalize_root so that ./a/./b normalizes to a/b.
• Introduce build_absolute_path, build_rooted_absolute_path, and build_relative_path as the canonical names; deprecate the old build_abs_path, build_rooted_abs_path, build_rel_path aliases.
• Use doctests when possible
• Update services to use the new function names.

Are there any user-facing changes?

No

AI Usage Statement

GPT-5.5 with codex implements some code.