[REF] XmlValidator: cache all_fields call

[cammarosano]

Performance analysis shows that Symbol::all_fields() is the most expensive function call during record validation. In order to avoid redundant recomputation when multiple records reference the same model in a single XML file, we cache results per model name.

Other changes for performance or code improvements:

• Deduplicate get_main_symbols call in validate_record
• Replace unnecessary diagnostic.clone() with moves
• Comment out no-op add_model_dependencies call (XmlFileSymbol not handled), and removed model_dependencies from method parameters
• Comment out mandatory_fields computation, which is not read anywhere
• Store module on XmlValidator struct, remove from all method parameters
• Rename validate_xml_id to validate_data for clarity (as we are not validating the XML ID itself, but the whole data entry)

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Measured improvements using tracing-timing (as per alpha-imp_perf_xml_validation-with-instrumentation-rcdl), running odoo_ls_server in --parse mode with community + enterprise 19.2:

Before
Timing histogram span="xml_validation" event="close" count=6687 p50_ms=0.147455 p90_ms=0.962559 p99_ms=8.126463 max_ms=149.946367 total_ms=4089.7898720000026

After
Timing histogram span="xml_validation" event="close" count=6687 p50_ms=0.112127 p90_ms=0.288767 p99_ms=0.724991 max_ms=5.963775 total_ms=968.1314160000005

Metric	Before	After	Change
count	6687	6687	—
p50 (ms)	0.147	0.112	-24%
p90 (ms)	0.963	0.289	-70%
p99 (ms)	8.126	0.725	-91%
max (ms)	149.95	5.96	-96%
total (ms)	4089.8	968.1	-76%