Fix prefix handling in Amazon transfer operators

[yuseok89]

This PR aligns filename prefix handling with the documented behavior of the Amazon FTP and SFTP transfer operators.

Previously, string prefixes were matched anywhere in a filename. This could select unintended files. Renaming also replaced every occurrence of the prefix, including occurrences later in the filename.

Documented behavior. Each operator's docstring already describes prefix semantics, not substring semantics:

:param s3_filenames: ... or a string prefix that all filenames must match.
:param ftp_filenames: ... prefix that replaces the s3 prefix.

The old implementation didn't match this. This PR makes the code match the docs, not the other way around.

The updated behavior:

• Selects only filenames that start with the configured source prefix.
• Replaces only the leading source prefix when a destination prefix is configured.
• Warns with the names of entries the previous substring rule selected but a prefix does not, naming up to ten.
• Matches and builds the destination key on the basename in FTP-to-S3, because some servers qualify nlst entries with the listed directory.
• Matches S3 keys on their file name, so nested keys under s3_key keep their directory at the destination.
• Applies consistently to FTP-to-S3, SFTP-to-S3, S3-to-FTP, and S3-to-SFTP transfers.
• Adds regression tests covering both file selection and prefix replacement.

Example, with ftp_filenames="pre_" / s3_filenames="new_":

Filename	Should match?	Before	After
pre_one.txt	yes	new_one.txt	new_one.txt
xpre_two.txt	no	selected (bug)	excluded
pre_again_pre_.txt	yes	new_again_new_.txt (corrupted mid-filename)	new_again_pre_.txt

Users relying on the undocumented substring-matching behavior may observe different file selection after this change.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  • gpt-5.6

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[vincbeck]

@ramitkataria @seanghaeli

[seanghaeli]

Looks good, pending merge conflicts

[seanghaeli]

Happy to merge this @o-nikolas @vincbeck @ferruzzi

edit: pending Ramit's below comments are addressed

[vincbeck]

Note for the next release manager, this a bug fix. It is a breaking change but considered as bug fix because the doc and code was not matching