fix(gomod): pseudo-version digest updates produce no change when module is absent from go proxy

[Maydaer]

Changes

Since Renovate version 43.167.0 (which includes PR #41588), digest updates for Go modules with pseudo-versions no longer produce file changes, if the repository is unavailable in the goproxy (for example repos on github enterprise instances, private repositories, etc).
The update process detects a newer commit, but only for newDigest.
The lookup for the new commit on proxy.golang.org fails, which causes upgrade.newValue in gomod.update to be equal to the current (old) value.

However, since current value is a pseudo-version, the version starts with v0.0.0-and gomod.update goes into the new code path

  if (upgrade.newValue?.startsWith('v0.0.0-')) {

added by the above PR, but since newVersion is the old version, no file changes occur.

This PR ensures the old behavior is triggered, if newValue and oldValue are identical, which handles the case where goproxy is unavailable.

Context

Please select one of the following:

• This closes an existing Issue, Closes: fix(gomod): pseudo-version digest updates produce no change when module is absent from go proxy #44455
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

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Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
  I used claude opus for the writing the test and the code change, but did some manual edits.
• Yes — other (please describe):

Documentation (please check one with an [x])

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I have verified these changes via:

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• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository:

[cla-assistant]

All committers have signed the CLA.

[RahulGautamSingh]

Else LGTM

[RahulGautamSingh]

Suggested change

// Should fall back to bare hash since newValue === currentValue

same as test name