bug: `bashunit doc` fails on missing default bootstrap instead of showing docs

[objctp]

Q	A
OS	macOS (also reproducible on Linux)
Shell & version	bash 3.2 / 5.2
bashunit version	0.44.0

Summary

bashunit doc fails with Error: cannot read the bootstrap file: 'tests/bootstrap.sh'. when no bootstrap file exists, instead of displaying the assertion documentation.

Current behavior

Running bashunit doc (the plain form, with no --custom and no --boot/-e/--env) in a project that does not yet have a tests/bootstrap.sh file prints an error and exits with code 1:

$ bashunit doc
Error: cannot read the bootstrap file: 'tests/bootstrap.sh'.

This happens even though the bootstrap is only needed to discover custom assertions. The default BASHUNIT_BOOTSTRAP value (tests/bootstrap.sh) is treated as mandatory before any documentation is printed.

How to reproduce

# Start from a clean project (no bootstrap file)
mkdir my-project && cd my-project
# (no tests/bootstrap.sh present)

# Plain documentation listing, as shown in the official docs:
$ bashunit doc [assert | contain]
# -> Error: cannot read the bootstrap file: 'tests/bootstrap.sh'.
#    exit code 1, no documentation shown

The same error also appears for bashunit doc --custom when no bootstrap exists (it should instead report "No custom assertions found.").

Expected behavior

Per the official doc documentation (https://bashunit.com/command-line#doc), bashunit doc "Displays documentation for assertion functions" and lists all assertions by default — no bootstrap file should be required.

• bashunit doc → prints all assertion documentation, regardless of whether a bootstrap file exists.
• bashunit doc --custom → prints "No custom assertions found." when no bootstrap/custom assertions are present (no hard error).
• Only an explicitly named bootstrap (bashunit doc --boot / -e / --env) should error when the file cannot be read.

[Chemaclass]

Done in #930 (merged).