FIX: Improve FreeSurfer ID predictability

[mgxd]

The intent after #3495 was encode as much information on how the anatomical template was formed into the FreeSurfer subject ID, but this introduced problems where the resulting FS ID could not be reused across a few cases:

• A new session was added after a previous run.
• Func-only processing leveraging precomputed derivatives from another session.

Starting this draft to think on how to cleanly address this in the future series.

Example 1

Assume we have sub-01 with a single session, pre:

--subject-anat-reference	FS ID (<25.2.x)	FS ID (25.2.x)	FS ID (26.0.x)
sessionwise	n/a	sub-01_ses-pre	sub-01_ses-pre
first-lex	sub-01	sub-01_ses-pre	sub-01
unbiased	sub-01	sub-01_ses-pre	sub-01

Now if a post session was added, the post's anatomical will be used in first-lex.

--subject-anat-reference	FS ID (<25.2.x)	FS ID (25.2.x)	FS ID (26.0.x)
sessionwise	n/a	sub-01_ses-pre, sub-01_ses-post	sub-01_ses-pre, sub-01_ses-post
first-lex	sub-01	sub-01_ses-post+pre	?
unbiased	sub-01	sub-01_ses-post+pre	?

IMO, sub-01 is misleading for both first-lex and unbiased, and only becomes hazier as additional sessions are added. Perhaps instead of changing the ID, some metadata (Sources) can be tied directly to the FreeSurfer subjects dir.

Example 2

sub-Z has one anat and many funcX sessions. anat was processed with --anat-only and will be reused via --derivatives

--subject-anat-reference	FS ID (<25.2.x)	FS ID (25.2.x)	FS ID (26.0.x)
sessionwise	n/a	func session, no anat found	n/a
first-lex	sub-Z	sub-Z_anat+func1+..	sub-Z
unbiased	sub-Z	sub-Z_anat+func1+..	sub-Z

Suggestions and additional examples are welcome and appreciated.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.18%. Comparing base (9f426f7) to head (a898448).

Files with missing lines	Patch %	Lines
fmriprep/interfaces/bids.py	88.88%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3607      +/-   ##
==========================================
- Coverage   73.19%   73.18%   -0.01%     
==========================================
  Files          60       60              
  Lines        4839     4834       -5     
  Branches      628      629       +1     
==========================================
- Hits         3542     3538       -4     
+ Misses       1156     1154       -2     
- Partials      141      142       +1     

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