[download] Fix SavePackage::TruncateBaseNameToFitPathConstraints

Resolves long-standing issues with filename truncation in SavePackage:

* Fix encoding-unaware truncation on POSIX systems by using UTF-8 safe
truncation with base::TruncateUTF8ToByteSize() to respect character
boundaries instead of unsafe byte-level substr().

* Replace hardcoded filesystem constants with runtime queries using
base::GetMaximumPathComponentLength() in
ComputeMaxPathLengthForDirectory() when blocking I/O is allowed (file
threads).

* Implement two-tier architecture: runtime filesystem queries during
path selection and conservative platform defaults during save operations
to avoid blocking I/O on UI thread.

* Add proper UTF-8 encoding conversion on POSIX (AsUTF8Unsafe/FromUTF8Unsafe)
with fallback handling for edge cases.

* Update comments and method names for clarity between blocking
(ComputeMaxPathLengthForDirectory) and non-blocking
(GetMaxPathLengthForDirectory) variants.

* Add comprehensive test coverage for UTF-8 truncation, edge cases, and
cross-platform compatibility including Fuchsia support.

Bug: 41258047
Change-Id: I9772618bc9eae0e48f0b85a6d0ccee82d057a276
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6809425
Commit-Queue: Ho Cheung <hocheung@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497291}