fix conversion from phase to range for covariance

[ZachHoppinen]

Description of proposed changes

This PR address issue #1340 which highlights that the conversion from phase to meters for covariance should be using a squared conversion factor in ifgram_inversion.py.

Reminders

• Fix Time-series covariance conversion #1340
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• Pass Circle CI test (green)
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Summary by Sourcery

Bug Fixes:

• Correct the covariance scaling factor when converting line-of-sight phase from radians to meters by applying the squared phase-to-range factor.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adjusts the phase-to-range conversion for covariance to correctly use the squared phase-to-range factor when converting from radians to meters in interferogram inversion.

File-Level Changes

Change	Details	Files
Correct phase-to-meter covariance scaling during interferogram inversion.	Update the time-series covariance scaling to use the square of the phase-to-range conversion factor when converting from LOS phase (radians) to meters, gated on covariance calculation. Keep existing logic for phase-to-range conversion and conditional covariance handling unchanged aside from the corrected scaling.	src/mintpy/ifgram_inversion.py

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#1340	Update the time-series covariance conversion from phase to range in ifgram_inversion.py so that it uses the squared phase-to-range conversion factor (i.e., multiply by	phase2range	^2 instead of

Possibly linked issues

• Time-series covariance conversion #1340: PR changes ts_cov multiplication to use |phase2range|², exactly addressing the covariance conversion issue.

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[sourcery-ai]

Hey - I've left some high level feedback:

• For clarity and to avoid any confusion about sign handling, consider using ts_cov *= phase2range**2 instead of np.abs(phase2range)**2, since the covariance scaling should inherently be sign-independent and phase2range is already defined with the appropriate sign.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- For clarity and to avoid any confusion about sign handling, consider using `ts_cov *= phase2range**2` instead of `np.abs(phase2range)**2`, since the covariance scaling should inherently be sign-independent and `phase2range` is already defined with the appropriate sign.

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[szwieback]

Can confirm this bug exists: our in-house code applies a post-hoc fix for it. This correction looks right and shouldn't cause side effects.