chore(ci): update merge script to match b0 beta suffix coming from main

[kryoseu]

Fixes: #11500

Problem

The beta path of merge_gradle.py searched theirs (main's app-thunderbird/build.gradle.kts) for versionNameSuffix = "b[1-9]". Since main carries b0, which b[1-9] never matches, so the search failed and the path raised SystemExit, causing git to mark the file as conflicted, so every main -> beta merge needed manual fixing.

Fix

Search for b\d+ (e.g b1, b2, b11, etc) so main's value matches whatever it is, and normalize the merge result to b0 for both apps. The "Bump version suffix" step in shippable_builds.yml bumps the suffix per Thunderbird beta release, so the merge result has to start at b0 for the first beta release to produce b1. We don't ship betas for k9, and the beta suffix is dropped on a release.

I've created a set_version_name_suffix() helper to be used for both branch logic, and two new constants: BETA_SUFFIX and MAIN_SUFFIX.

Three details:

• The search now targets ours rather than theirs. shutil.copyfile(theirs, ours) runs first, so the two are identical in content, but ours is the file being patched and the one whose final state matters.
• SystemExit is now reserved for the case where the searched suffix line doesn't exist at all: a1 on the app-k9mail path, b\d+ on app-thunderbird.
• The release path uses b\d+ too, replacing b[1-9], so it no longer breaks once a cycle reaches b10.

Testing

Ran the driver directly against the pre-merge branch tips:

	ours (beta)	theirs (main)	result
app-thunderbird	23.0 / code 58 / b1	24.0 / code 4 / b0	24.0 / code 58 / b0
app-k9mail	23.0 / code 39021 / b1	24.0 / code 39004 / a1	24.0 / code 39021 / b0

[github-actions]

✅ Validation Passed: All report and feature-flag labels are correctly set.

[coreycb]

We might actually be able to remove the is_k9 code block since we don't release beta for k9. Or if you want to keep it you can probably re-use more of the code across k9/thunderbird paths below.

[kryoseu]

If we remove the is_k9 code block, beta will inherit a1 from main. While that's fine for beta, it will flow into the next beta -> release merge and break the release path in the script.

[kryoseu]

I will update the script to re-use more of the code though.

[coreycb]

We might be able to get away with b\d since I think we generally don't get past b3 or b4. Or maybe b\d{1,2}(?!\d) if we want to include 2 digits.

[kryoseu]

The current regex (with d+) does that:

>>> import re
>>> pat = r'versionNameSuffix = \"b\d+\"'
>>> for line in ['versionNameSuffix = "b1"', 'versionNameSuffix = "b2"', 'versionNameSuffix = "b11"', 'versionNameSuffix = "a1"']:
...  print(line, '->', bool(re.search(pat, line)))
...
versionNameSuffix = "b1" -> True
versionNameSuffix = "b2" -> True
versionNameSuffix = "b11" -> True
versionNameSuffix = "a1" -> False

[coreycb]

That's fine. I was just thinking it might be too much because we'll never have a b000009. Should be fine though.

[coreycb]

Looks good, thank you