SFTP Bulk: CSV header detection can silently skip initial records when remote reads do not honor rewind

## Summary

The SFTP Bulk connector can complete a CSV sync successfully while emitting fewer records than the source file contains.

The discrepancy occurs with **CSV Header Definition: From CSV**. Switching to **User Provided** headers and setting **Skip Rows Before Header: 1** restores the expected record count.

Source-code analysis and an offline reproduction indicate that the header-reading implementation depends on the SFTP server correctly honoring a rewind. If subsequent reads continue from the previous server-side position instead, initial data records can be silently skipped.

The workaround is confirmed against the affected source. The remote server's offset handling has not yet been independently verified.

## Versions investigated

- `source-sftp-bulk`: **1.9.2**
- Airbyte CDK: **7.5.1**, from the connector's dependency lockfile
- Paramiko: **3.4.0**

## Observed behavior

- The CSV's data-record count was independently verified using Python's CSV parser.
- Every record had the expected number of fields.
- The file contained no blank records or repeated headers.
- The source emitted fewer records than expected.
- The destination committed every emitted record.
- The sync completed without reported parsing failures.
- Providing explicit headers and skipping the physical header restored the full count.

## Suspected mechanism

With **From CSV** headers, the CDK CSV reader:

1. Reads the header from the remote file.
2. Calls `seek(0)`.
3. Skips the header line.
4. Parses the remaining records.

Paramiko reads ahead in **8,192-byte blocks** when reading lines. Its `seek(0)` clears the read buffer and causes the next SFTP READ request to use offset zero.

If the server or gateway ignores that offset and continues from its previous position:

- The buffered initial records are lost.
- The subsequent header-skipping step discards the remainder of the line at the block boundary.
- Parsing continues with valid later records, allowing the sync to report success.

## Offline reproduction

Using the actual Paramiko 3.4.0 file implementation, a local CSV, and a simulated SFTP backend:

| Backend behavior | Header mode | Result |
|---|---|---|
| Honors requested offsets | From CSV | All records emitted |
| Ignores requested offsets | From CSV | Initial records silently omitted |
| Ignores requested offsets | User Provided, skip one line | All records emitted |

The omitted records in the reproduction were exactly the initial records consumed by the read-ahead and subsequent line skip.

This does **not** imply that all SFTP servers are affected.

## Expected behavior

The connector should either:

- Emit all CSV data records; or
- Fail clearly when the remote file cannot be rewound reliably.

A successful sync should not silently omit initial records.

## Workaround

- Set **CSV Header Definition** to **User Provided**.
- Supply the column names in file order.
- Set **Skip Rows Before Header** to **1**.
- Leave **Skip Rows After Header** at **0**.

This avoids the header-detection rewind.

## Suggested improvement

Consider parsing the header and data in a single forward-only pass, preserving the existing buffer instead of rewinding the remote file.

If rewinding remains necessary, consider validating that the rewind succeeded and surfacing an explicit error otherwise.

## Relevant code

- [CDK CSV parser](https://github.com/airbytehq/airbyte-python-cdk/blob/v7.5.1/airbyte_cdk/sources/file_based/file_types/csv_parser.py)
- [Paramiko SFTP read and seek implementation](https://github.com/paramiko/paramiko/blob/3.4.0/paramiko/sftp_file.py)
- [Paramiko buffered line reader](https://github.com/paramiko/paramiko/blob/3.4.0/paramiko/file.py)

Start by reading the CDK CSV parser at airbyte_cdk/sources/file_based/file_types/csv_parser.py, then inspect Paramiko's sftp_file.py and file.py implementations for the read and rewind behavior described. Reproduce the offset-ignoring backend case from the issue and determine whether the completed change emits every record or fails clearly when rewind cannot be trusted.

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