mk_sap: apply host_prefix to logwatch piggyback

[Simon0Harms]

General information

Affected plugin: agents/plugins/mk_sap.py — the SAP R/3 agent plugin that
collects CCMS data via RFC (pyrfc) and emits it as piggyback blocks keyed by the
SAP System ID (SID). The host_prefix option prepends a string to the piggyback
host name.

This is a platform-independent issue (a Python string-formatting inconsistency);
it was observed on SLES 15 SP7 with pyrfc and NW RFC SDK 7.50, but does not depend
on the OS.

Bug report

• Operating system: SLES 15 SP7 (issue is OS-independent)

• Local setup: two SAP systems sharing the same SID (an ECC and an S/4HANA system).
  host_prefix is used to give each source's piggyback data a distinct host name.

• Steps to reproduce:

  1. Set host_prefix in /etc/check_mk/sap.cfg, e.g. "host_prefix": "S4H_".
  2. Run the plugin so that it produces both a <<<sap>>> section and a
     <<<logwatch>>> section (i.e. at least one MTE_MSG_CONTAINER with alerts).
  3. Inspect the emitted piggyback headers.

• Agent output (illustrative, with host_prefix = "S4H_"):

  Observed (current behavior):

<<<<S4H_SB1>>>>
<<sap:sep(9)>>

<<<<>>>>
<<<<SB1>>>>            <-- logwatch header WITHOUT the prefix
<<<logwatch>>>

The SAP section and the logwatch section end up on two different piggyback
hosts (S4H_SB1 vs. SB1).

Proposed changes

• Expected behavior: with host_prefix set, both the sap section and the
  logwatch section are written under the same (prefixed) piggyback host.
• Observed behavior: only the SAP data header includes the prefix; the logwatch
  header omits it. In check(), the SAP block writes
  "<<<<%s%s>>>>\n" % (cfg_entry.get("host_prefix", ""), host) while the logwatch
  block writes "<<<<%s>>>>\n" % host, dropping the prefix.
• Change: apply the same host_prefix formatting to the logwatch header so both
  sections share one piggyback host.
• Is this a new problem? No — a longstanding inconsistency. It surfaces when
  host_prefix is used, which is the natural way to disambiguate systems that
  share a SID.
• Unit test: a test that runs the output stage with a non-empty host_prefix and
  asserts that both the sap and the logwatch piggyback headers carry the
  prefix would have failed before this fix.

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[Simon0Harms]

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