DAOS-18304 ddb: Fix memory leaks, defer ordering, and pool handle corruption

[knard38]

Description

This PR is the first split of #17967, broken out at reviewers' request to make the review process easier. It contains unrelated code fixes found during the creation of the initial PR. The second split #18124 contains the main refactoring and will be reviewed separately.

Go layer

• Feature(): C.CString(enable) and C.CString(disable) were passed directly to ddb_feature_string2flags() without ever being freed; assign them to named variables so defer freeString() can release them.
• DtxStat(): options.details was set before defer freeString(options.path), inverting the defer execution order; move the defer immediately after the CString allocation.

C layer

• ddb_run_feature(): dc_poh and dc_write_mode were reset unconditionally, corrupting the caller's pool state when the pool was already open before the call; guard both resets inside the if (close) block.
• helper_stat_open_modify_close_stat(): the function overwrote tctx->dvt_poh with a temporary handle and never restored it, leaving the suite-level handle invalid for subsequent tests; save and restore the original handle around the open/close sequence.
• read_only_vs_write_mode_test(): assert_memory_equal() used fs[FILE_STATE_PRE].digest as both arguments, making the read-only assertion a no-op; use fs[FILE_STATE_POST].digest as the second argument.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'Add unit test to ddb go code'
Status is 'In Review'
https://daosio.atlassian.net/browse/DAOS-18304

[Nasf-Fan]

Good catching them!

[knard38]

@daos-stack/daos-gatekeeper could you lend this PR. The only pending CI step has been skipped by the CI itself. If possible, I would like to avoid to restart a CI round as it is very unstable lately. If it is OK, could you lend it with this message:

• title: DAOS-18304 ddb: Fix memory leaks, defer ordering, and pool handle corruption
• message:

   Go layer:
   - Feature(): assign C.CString(enable/disable) to named variables so
     defer freeString() can release them; previously both CStrings were
     passed directly to ddb_feature_string2flags() and never freed.
   - DtxStat(): move defer freeString(options.path) immediately after the
     CString allocation, before options.details is set, to restore correct
     defer execution order.

   C layer:
   - ddb_run_feature(): guard dc_poh and dc_write_mode resets inside the
     if (close) block to avoid corrupting the caller's pool state when the
     pool was already open before the call.
   - helper_stat_open_modify_close_stat(): save and restore tctx->dvt_poh
     around the open/close sequence so the suite-level handle remains valid
     for subsequent tests.
   - read_only_vs_write_mode_test(): fix assert_memory_equal() that used
     fs[FILE_STATE_PRE].digest as both arguments, making the read-only
     assertion a no-op; use fs[FILE_STATE_POST].digest as second argument.

[tanabarr]

go changes LGTM