Add SRU 2.0 support (backwards compatible)

[Copilot]

SRU 2.0 uses different XML namespaces than SRU 1.x for response, explain, and diagnostics elements. The library had no awareness of these namespaces, causing SRU 2.0 server responses to fail parsing entirely.

Changes

• xmlparse.py: Register SRU 2.0 namespaces (sruResponse, sruExplain) in both namespaces and dict_namespaces, mapping them to the existing "sru" key so all downstream XPath queries work without modification.
• response.py: Refactored _check_response_tag to detect SRU 2.0 root namespaces and hot-swap xmlparser.namespaces["sru"] accordingly, making version detection transparent to all response parsing logic.
• client.py: Extended _check_errors to also probe the SRU 2.0 diagnostics namespace (http://docs.oasis-open.org/ns/search-ws/diagnostic). Added a sort_keys parameter to searchretrieve() that maps to the sortKeys URL parameter for SRU 1.x requests; the parameter is silently omitted for SRU 2.0, where sorting is done via the CQL sortby clause in the query.
• __init__.py: Exposed sort_keys as a passthrough parameter in the module-level searchretrieve() function.
• README.md: Updated to advertise SRU 1.1, 1.2, and 2.0 support.

Usage

# SRU 2.0 endpoint — just pass sru_version="2.0"
client = sruthi.Client("https://example.com/sru", sru_version="2.0")
results = client.searchretrieve("query")  # sruResponse namespace handled automatically

info = client.explain()  # sruExplain namespace handled automatically
print(info.sru_version)  # "2.0"

# SRU 1.x sorting via sortKeys parameter
results = client_1x.searchretrieve("query", sort_keys="title,,0")

# SRU 2.0 sorting via CQL sortby clause (sortKeys is ignored)
results = client_2x.searchretrieve("query sortby title/sort.ascending")

SRU 1.1 and 1.2 behavior is unchanged.

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[metaodi]

@copilot is everything listed here addressed in this PR? E.g. the new sortKeys parameter.

[Copilot]

@copilot is everything listed here addressed in this PR? E.g. the new sortKeys parameter.

The differences page wasn't reachable from this environment, but based on the SRU 2.0 specification the relevant differences for a client library are:

• Namespaces (sruResponse, sruExplain, diagnostic) — handled ✅
• sortKeys parameter — sortKeys is a SRU 1.x-only request parameter (in SRU 2.0, sorting is done via the CQL sortby clause in the query). I've added a sort_keys parameter to searchretrieve() that emits sortKeys for SRU 1.x and silently omits it for SRU 2.0 (commit e0cb958).
• recordXMLEscaping vs recordPacking — renamed in SRU 2.0 responses, but since the parsing goes directly to recordData, this doesn't affect functionality ✅
• operation parameter — optional in SRU 2.0 but still accepted by all known implementations; we keep sending it ✅
• README — updated to advertise SRU 2.0 support ✅

If there's a specific item from that page I missed, happy to address it.

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