[io] fix TFileMerger kOnlyListed suffix key leak

[prakharsing7]

When using PartialMerge with kOnlyListed an unlisted key whose name is a prefix or suffix of a listed key was incorrectly included in the merged output, because the name check was a plain substring search with no leading word boundary.

This Pull request:

Changes or fixes: #22414

Checklist:

• tested changes locally
• updated the docs (if necessary)

[pcanal]

Thanks.

[Copilot]

Pull request overview

This PR fixes TFileMerger::kOnlyListed object selection so listed object names are matched as space-delimited tokens, preventing unlisted keys whose names are suffixes of listed keys from being merged.

Changes:

• Updates TFileMerger::MergeOne to search for " key " within a space-prefixed object-name list.
• Adds a regression test covering suffix/prefix-like names when using kOnlyListed.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
io/io/src/TFileMerger.cxx	Tightens kOnlyListed name matching to require a leading delimiter.
io/io/test/TFileMergerTests.cxx	Adds regression coverage ensuring only the explicitly listed key is merged.

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[github-actions]

Test Results

    22 files      22 suites   3d 11h 12m 57s ⏱️
 3 863 tests  3 862 ✅ 0 💤 1 ❌
76 179 runs  76 178 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 3379b33.

♻️ This comment has been updated with latest results.

[guitargeek]

/backport to 6.40

[root-project-bot]

Preparing to backport PR #22435 to branch 6.40 requested by guitargeek

[root-project-bot]

This PR has been backported to branch 6.40: #22445