[Backport 2025.3] fix(config): prevent billing_project set to "staging" in Python and Jenkins code

[scylladbbot]

Description

Commit 5063a93 introduced billing_project extraction from JOB_NAME but has bugs in both Python and Jenkins Groovy code:

1. Sets billing_project="staging" when JOB_NAME is scylla-staging/* or scylladb-staging/*
2. Creates sct-runner instances with empty billing_project="" tag
3. Jenkins tagBuilder.groovy has regex bug and missing staging check
4. TestConfig.common_tags() reads billing_project directly from environment instead of using proper configuration logic

Changes:

sdcm/sct_config.py:

• Skip setting billing_project when extracted value equals "staging"
• Fix log message to show actual billing_project value instead of prefixed release_folder

sdcm/test_config.py:

• Use SCTConfiguration to get billing_project when cls._tester_obj doesn't exist
• This ensures proper logic including JOB_NAME parsing, staging exclusion, and git branch fallback
• Falls back to environment variable only if configuration creation fails (instead of always reading directly from os.environ)

vars/tagBuilder.groovy:

• Fix regex matching operator from == to =~ for proper pattern matching
• Add check to prevent setting billing_project to "staging" (same logic as Python)
• Fix echo statement to use correct variable name (billingProjectValue instead of undefined part)

Example:

# Before: JOB_NAME="scylla-staging/longevity/test" → billing_project="staging"
# After:  JOB_NAME="scylla-staging/longevity/test" → billing_project not set

# Before: SCT_BILLING_PROJECT="" → tags["billing_project"]=""  
# After:  SCT_BILLING_PROJECT="" → "billing_project" key not in tags

# Before: common_tags() reads SCT_BILLING_PROJECT directly from os.environ
# After:  common_tags() uses SCTConfiguration for proper JOB_NAME/git branch logic

Testing

• Unit tests added: test_39_billing_project_from_job_name, test_39_billing_project_staging_not_set

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

Original prompt

5063a93 has a small bug, it can set billing_project to staging base on jenkins folder, it shouldn't set it if it's staging, also seem like sct-runner are created with empty billing_project, try finding where and address it

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

• (cherry picked from commit 0fa7de0)

Parent PR: #13268

[scylladbbot]

@copilot - This PR has conflicts, therefore it was moved to draft
Please resolve them and mark this PR as ready for review