Type plan_force_flat ids as bigint in @expert_mode parser

[erikdarlingdata]

Refs #767, follow-up to #771.

Summary

• The string-split path that parses sys.dm_db_tuning_recommendations.details (the pre-2017-compat fallback) returns regressed_plan_id / recommended_plan_id with a leading space — the REPLACE chain inserts ': ' after every colon, and the SUBSTRING math doesn't account for it.
• The original code relied on the implicit cast on insert into #tuning_recommendations (defined as bigint NULL) to silently strip the space. PR Fix: error while inserting #tuning_recommendations #771 added TRY_CAST(... AS bigint) in the WHERE clause to survive the same surface in another spot.
• This change moves the type intent to the projection: TRY_CAST(LTRIM(SUBSTRING(...)) AS bigint) so the derived-table columns leave the parser already typed.
• Drops the now-redundant TRY_CAST from the WHERE clause that Fix: error while inserting #tuning_recommendations #771 added.
• Does not touch the SUBSTRING/CHARINDEX/IIF math — that lattice is fragile and the leading-space symptom isn't actually a math bug, it's a REPLACE side-effect.

Credit: @ClaudioESSilva confirmed via repro on Hyperscale that no truncation is happening, just the leading space, and the implicit cast was the silent rescuer.

Test plan

• Installed updated proc on SQL2022.
• EXEC dbo.sp_QuickieStore @expert_mode = 1, @top = 1 against StackOverflow2013 — no conversion error, no 8114/8145.
• Repro tenant on Azure SQL DB Hyperscale (per @ravirajch / @ClaudioESSilva) — would appreciate a confirmation run.

[ClaudioESSilva]

@erikdarlingdata compiled and executed without error!

[erikdarlingdata]

@ClaudioESSilva first time for everything 😛