fix TdtRawIO multi-block SEV file lookup (wrong path + case mismatch)

[ree-gupta]

Fixes multi-block SEV file lookup in TdtRawIO. Two bugs caused silent fallback to TEV, resulting in all channels reading identical data:

1. path variable at L290 was stale (from iterdir() loop at L103 or reassigned to tank root at L170), never pointed to the block subdirectory where SEV files live
2. Filename used lowercase ch but TDT writes Ch, not found on case-sensitive filesystems (Linux)

Fix: use self.dirname / segment_name for the correct block directory, and [cC]h glob to match both cases (same pattern the single-block path already uses).

Not caught by tests because aep_05 (the only multi-block test dataset) has no SEV files.

Closes #1814

[pauladkisson]

I can confirm that this fix also works for my use case (hidden ._ files/folders)

[h-mayorquin]

LGTM.

One small comment and I am fine with this if the tests pass : )

[h-mayorquin]

I find the None strange here. Did you copied this from the segment branch, correct?

I think this is a mistake (see here #1817)

Given that you are using a fully qualified name here finding two files should throw an error.

Maybe something like this would be better

sev_filename = list(block_path.glob(sev_regex))
if len(sev_filename) == 1:
    sev_filename = sev_filename[0]
elif len(sev_filename) == 0:
    sev_filename = None   # Indirect flag for TEV Format, see issue 1087
else:
    raise ValueError(f"Multiple SEV files matched for channel {chan_id}: {sev_filename}")

[ree-gupta]

Ah, thanks, I didnt know the that the format only allows either sev or tsv. Updated the PR with your suggestion, please have a look :)

[ree-gupta]

One minute, made an indentation error

[ree-gupta]

One small comment and I am fine with this if the tests pass : )

The available tests for this file pass locally :)

[h-mayorquin]

@alejoe91 @samuelgarcia @zm711
This looks good to me. I think is a simple fix to an obvious error. Let me know if you want to take a look otherwise it would be good to merge so I can work on:

#1817