cvd host_bugreport: don't discard the whole archive on a directory entry

[shuowanghsu]

AddFileAt guarded its input only with FileExists, which is a stat check and therefore succeeds for anything that exists, including directories. libzip does not read the source until WritableZip::Finalize, where reading a directory fails with EISDIR. libzip then calls zip_source_rollback_write, which discards the entire archive rather than just the offending entry.

AddNetsimdLogs hits this: it lists the netsimd temp directory with a non-recursive DirectoryContents, so when netsimd runs with --pcap it hands the pcap/ directory straight to AddFileAt. The result is that cvd host_bugreport produces an archive containing almost nothing, and the netsimd capture files are never collected either.

Against the unpatched library the new test reproduces it exactly:

Expected "zip_close(raw_zip)" == "0" but was -1 vs 0. Read error: Is a directory

Changes

1. Require a regular file in AddFileAt. This is the important one — today any caller that passes a directory silently destroys the archive it was building, and does so at finalization time, far from the offending call site. After this change a bad entry can only lose itself. The check is S_ISREG rather than just a directory test, because a FIFO or a socket reaches Finalize the same way, and opening one there can fail or block indefinitely.
2. Collect netsimd logs recursively, naming each entry by its path relative to the netsimd directory, so the capture files under pcap/ are actually included. Per-entry errors are logged rather than returned, so one bad file cannot abort the rest of the walk.
3. Add WalkDirectoryFiles next to the existing WalkDirectory in common/libs/utils/files.h, reporting only non-directories. Symlinks are not followed when deciding whether to recurse, so a symlink loop cannot trap the walk.
4. Add regression tests for both halves: that AddFileAt rejects directories and FIFOs, that a rejected entry no longer takes unrelated entries down with it at Finalize, and that WalkDirectoryFiles recurses while WalkDirectory keeps reporting directories.

Notes

• WalkDirectoryFiles is a new sibling rather than a change to WalkDirectory, because SymlinkHostPackage, WriteFsConfig and LinkOrCopyDirectoryContentsRecursively all rely on WalkDirectory invoking the callback for directories. WriteFsConfig in particular would silently drop directory entries from filesystem_config.txt. Happy to consolidate them differently if you'd prefer.
• logs/, tombstones/ and recording/ are left non-recursive. They are flat in practice, and after change 1 an unexpected subdirectory there is merely logged and skipped rather than fatal. Can extend WalkDirectoryFiles to them if you'd like uniformity.
• Per-entry errors remain non-fatal, consistent with the existing contract that "a partial bug report is still useful."

Testing

bazel test  //cuttlefish/host/libs/zip:zip_file_test \
            //cuttlefish/common/libs/utils:files_test
bazel build //cuttlefish/host/commands/host_bugreport:cvd_internal_host_bugreport

All new test cases pass with the fix; the zip_file_test cases fail without it.