refactor: consolidate map utilities, deep clone, and logger constant

[Copilot]

Four concrete code quality issues found via semantic function clustering: a misplaced utility, a near-duplicate private helper, a single-function package, and a dead constant alias.

Finding 1+2 — GetStringFromMap relocated and generalized; getFilteredItemStringField removed

GetStringFromMap was living in strutil/truncate.go with no relation to truncation. Moved to new strutil/maputil.go and upgraded to variadic keys with first-non-empty semantics — which exactly matches what server/difc_log.go's private getFilteredItemStringField was doing:

// Before: private copy in difc_log.go with a TODO comment to consolidate
entry.AuthorAssociation = getFilteredItemStringField(m, "author_association", "authorAssociation")

// After: shared utility handles camelCase/snake_case fallback directly
entry.AuthorAssociation = strutil.GetStringFromMap(m, "author_association", "authorAssociation")

Single-key callers in proxy/ are unaffected (identical behaviour for the non-empty case).

Finding 3 — internal/jsonutil/ package deleted

jsonutil.DeepCloneJSON was the only function in the package and had exactly one production caller (proxy/response_transform.go). Moved to internal/strutil/deepclone.go alongside the other general-purpose utilities. Import updated in response_transform.go and the three proxy test files.

Finding 4 — Dead logTimestampLayout alias removed

// Before: confusing two-constant indirection
logTimestampLayout  = "2006-01-02T15:04:05.000Z07:00"  // never referenced directly
jsonTimestampLayout = logTimestampLayout

// After: single constant
jsonTimestampLayout = "2006-01-02T15:04:05.000Z07:00"

[Copilot]

Pull request overview

This PR refactors internal utilities by consolidating JSON deep-cloning and untyped map string extraction into internal/strutil, removing a now-redundant helper in server, deleting the single-function jsonutil package, and simplifying a logger timestamp constant.

Changes:

• Moved and generalized GetStringFromMap into internal/strutil/maputil.go (variadic keys, first non-empty semantics) and replaced server’s private helper usage.
• Moved DeepCloneJSON into internal/strutil and updated proxy code/tests to use strutil.DeepCloneJSON; removed the internal/jsonutil package tests.
• Removed the unused logTimestampLayout alias in internal/logger/jsonl_logger.go.

Show a summary per file

File	Description
internal/strutil/util_test.go	Expanded GetStringFromMap unit tests to cover new variadic semantics.
internal/strutil/truncate.go	Removed misplaced GetStringFromMap from truncation utilities.
internal/strutil/maputil.go	Introduced consolidated GetStringFromMap (variadic keys, first non-empty).
internal/strutil/deepclone.go	Relocated DeepCloneJSON into strutil.
internal/server/difc_log.go	Replaced local string-field helper with strutil.GetStringFromMap.
internal/server/difc_log_helpers_test.go	Removed tests for deleted helper; retained other helper tests.
internal/proxy/response_transform.go	Switched deep-clone call from jsonutil to strutil.
internal/proxy/response_transform_test.go	Updated deep-clone tests to use strutil.DeepCloneJSON.
internal/proxy/response_transform_coverage_test.go	Updated coverage tests to use strutil.DeepCloneJSON.
internal/proxy/proxy_test.go	Updated deep-clone test to use strutil.DeepCloneJSON.
internal/logger/jsonl_logger.go	Simplified timestamp layout constant definition.
internal/jsonutil/clone_test.go	Removed tests as internal/jsonutil package is deleted.

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• Files reviewed: 12/12 changed files
• Comments generated: 2