perf(private_key): look up file size once per file instead of per line

[Saarett]

PrivateKeyDetector.analyze_line runs for every line of every scanned file. The file-size guard was written as a single and expression:

if filename not in self._analyzed_files \
        and 0 < self.get_file_size(filename) < MAX_FILE_SIZE:
    self._analyzed_files.add(filename)

_analyzed_files was only populated when the size fell inside the scannable range. For any file at or above MAX_FILE_SIZE (8 KiB) the membership check kept failing, so get_file_size -> os.path.getsize fired again on every single line: a per-file operation executed per-line. Cost scaled with (files x lines) rather than (files).

Track the files whose size has already been measured in a separate _sized_files set, so the lookup happens at most once per file regardless of the result. The subsequent whole-file read is unchanged and still happens exactly once per file, so multi-line private keys are detected exactly as before.

Measured on a 21,359-file repository via
checkov --framework secrets --enable-secret-scan-all-files:

before:  real 2120s   user 2014s   sys 230s
after:   real 1679s   user 1638s   sys  60s

Wall time -21%; sys time (the syscall fingerprint of the redundant getsize calls) down 3.9x. Findings are identical before and after (14 findings, same files/lines/checks).

Adds two regression tests:

• test_get_file_size_is_called_at_most_once_per_file - asserts the size lookup runs at most once for a 500-line, >8 KiB file (previously 500 times).
• test_multiline_private_key_in_small_file_is_still_detected - guards the whole-file read path that multi-line key detection depends on.

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