refactor: resolve duplicate string literals, mutable default arguments, and type mismatches in Python scripts

[bhagathkrishnacdac]

Overview
This PR addresses several critical code quality issues across the project's Python files (under bessctl/, bin/, and build.py), focusing on string deduplication, correct parameter initialization, and type safety.

Key Changes

• Eliminated Mutable Default Arguments: Resolved a classic Python code smell in bessctl/measurement_utils.py by changing a mutable default parameter value to None and initializing it within the function block to prevent shared state bugs.
• Consolidated Duplicate String Literals: Defined constants for repeated string literals across test suites, daemon management scripts, and devbind tools (e.g., duplicated log messages, interface formats, and testing MAC/URL strings).
• Corrected Subprocess Argument Types: Resolved a typing mismatch in bessctl/commands.py where the argument passed to check_call did not match the expected type (e.g., passing a string instead of a list of arguments, or vice versa).
  Verification

These changes do not alter any external behaviors, console outputs, or build pipeline results. All changes represent standard, non-functional code quality improvements.

[gab-arrobo]

Some minor changes due to warnings from git when applying patch. Also, some other minor format changes

$ git apply pr.patch
pr.patch:187: trailing whitespace.
        if rtt_percentiles is None:
pr.patch:234: trailing whitespace.
BLACKLISTED_HOST = 'www.blacklisted.com'
pr.patch:349: trailing whitespace.
UNBIND_OPEN_ERROR = "Error: unbind failed for %s - Cannot open %s"
pr.patch:413: trailing whitespace.
# constants for duplicate literals
warning: 4 lines add whitespace errors.

[gab-arrobo]

Suggested change

	BLACKLISTED_HOST = 'www.blacklisted.com'
	BLACKLISTED_HOST = 'www.blacklisted.com'

[gab-arrobo]

Suggested change

	UNBIND_OPEN_ERROR = "Error: unbind failed for %s - Cannot open %s"
	UNBIND_OPEN_ERROR = "Error: unbind failed for %s - Cannot open %s"

[gab-arrobo]

Suggested change

	# constants for duplicate literals
	# constants for duplicate literals

[gab-arrobo]

Suggested change

	if rtt_percentiles is None:
	if rtt_percentiles is None:

[gab-arrobo]

Should this become a single line?

Suggested change

	print(UNBIND_OPEN_ERROR
	% (dev_id, filename))
	print(UNBIND_OPEN_ERROR % (dev_id, filename))

[gab-arrobo]

Suggested change

	print(UNBIND_OPEN_ERROR
	% (dev_id, filename))
	print(UNBIND_OPEN_ERROR % (dev_id, filename))

[gab-arrobo]

Suggested change

	print(UNBIND_OPEN_ERROR
	% (dev_id, filename))
	print(UNBIND_OPEN_ERROR % (dev_id, filename))

[Copilot]

Pull request overview

This PR refactors several Python scripts/tests to reduce repeated string literals, fix a mutable default argument, and tighten up a subprocess invocation parameter type, aiming to improve code quality without changing intended behavior.

Changes:

• Introduces constants to replace duplicated string literals across build tooling, CLI output, and module tests.
• Fixes a mutable default argument in PortStatsGenerator by switching from list() to None + in-body initialization.
• Adjusts subprocess.check_call(..., shell=...) to use a boolean True instead of the string 'True'.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
build.py	Adds a constant for the builtin protobuf output directory and reuses it in generation/cleanup paths.
bin/dpdk-devbind.py	Factors repeated error/help strings into constants and reuses them across error paths.
bessctl/test_samples.py	Introduces a constant for the daemon start command string used across test setup/recovery.
bessctl/sugar.py	Introduces a constant for the __bess_env__ prefix used in environment-variable substitution strings.
bessctl/module_tests/url_filter.py	Replaces repeated host literals with constants in URL filter tests.
bessctl/module_tests/arp.py	Replaces repeated MAC address literals with a constant in ARP responder tests.
bessctl/measurement_utils.py	Fixes a mutable default argument by initializing rtt_percentiles safely per instance.
bessctl/commands.py	Consolidates repeated CLI output strings into constants and fixes a shell parameter type.

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